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The MC@NLO 4.0 Event Generator&lowast;

1. only in tt Wt WtW and W Z production are assigned certain values In the case of individual lepton decays these range from 1 to 3 if the decaying particle is a W or a top or from 1 to 6 if the decaying particle is a Z For these cases the value of ILa fully determines the identity of the leptons emerging from the decay and the same convention as in HW6 is adopted see the HW6 manual and sect 1 2 In tt and single top production i e for all processes listed in table 2 which involve t and or t the top quark and or antiquark and the hard W in the case of Wt production can also decay hadronically In such cases therefore the variables ILg can be assigned more values than for the other processes the situation is summarized in table 3 When generating the decays lepton and flavour universalities are assumed The relative probabilities of individual hadronic decays e g Wt ud vs Wt u5 are determined using the CKM matrix elements entered by the user variables Vud in MCatNLO inputs The relative probabilities of leptonic vs hadronic decays are on the other hand determined using the values of the corresponding branching ratios entered by the user variables BRTOPTOLEP and BRTOPTOHAD for top antitop decays and BRWTOLEP and BRWTOHAD for the decays of the hard W emerging from the hard process in Wt production see eqs 4 1 and 4 2 for Non factorizable spin correlations of virtual origin are not included in WtW tt and
2. PDF group 2 PDFSET PDF id number 2 LAMBDAHERW Lambda_5 lt 0 for default WMASS WMASS ZMASS M_W M_W MZ UMASS DMASS SMASS CMASS BMASS GMASS quark and gluon masses Table 6 Sample input file for the MC code for vector boson pair production resulting from setting HERPDF EXTPDF which implies pdftype 1 Setting HERPDF DEFAULT results in an analogous file with pdftype 0 and without the lines concerning PDFGROUP and PDFSET EVPREFIX must be understood with SCRTCH in front see sect 5 The negative sign of IPROC tells the EvG to use Les Houches interface routines Variable Default value esctype 0 pdftype 0 1 HERPDF DEFAULT EXTPDF beammom EMC 2 Table 7 Default values for script generated variables in MCinput In the case of y Z W Higgs or heavy quark production the MC executable can be run with the corresponding positive input process codes IPROC 1350 1399 1499 1600 ID 1705 1706 2000 2008 2600 ID or 2700 ID to generate a standard HW6 run 12 for comparison purposes Then the input event file will not be read instead parton configurations will be generated by HW6 according to the LO matrix elements References 1 S Frixione and B R Webber Matching NLO QCD computations and parton shower simulations JHEP 0206 2002 029 hep ph 0204244 2 S Frixione P Nason and B R Webber Matching NLO QCD and parton showers in heavy flavour production JHEP 0308 2003 007 arXiv
3. herwig65nn inc with 65nn the version chosen by the user this must be consistent with the value of the input parameter HERWIGVER see sects 4 and 5 The file mcatnlo_hwdriver f contains a set of read statements which are necessary for HW6 to get the input parameters see sect 4 for the input procedure these read statements must not be modified or eliminated Also mcatnlo_hwdriver f calls the HW6 routines which perform showering hadronization decays see sect 4 6 for more details on this issue and so forth the user can freely modify this part as is customary in HW6 runs Finally the sample codes mcatnlo_hwanzzz f and mcatnlo_hwanzez_rb f contain analysis related routines these files must be replaced by files which contain the user s analysis routines We point out that since version 2 0 the Makefile need not be edited any longer since the corresponding operations are now performed by setting script variables see sect 5 3 2 Usage with HW When using HW no editing of the source codes is required except for those related to analysis to be found in the directory HWppAnalyzer The analogues of the read state ments of HW6 mcatnlo_hwdriver f are in the case of HW the settings of parame ters in HWPPInput inputs which the user can modify at will However the user will have to provide installed versions of HW and ThePEG 31 We advise the user to adopt versions 2 4 2 for HW and 1 6 1 for ThePEG or later We stres
4. Qem in lepton pair and single vector boson production set it to NO to use Qem 1 137 0359895 Set this variable equal to 1 or 2 in order to use a type I or a type II 2HDM model for the computation of H t production Set this variables equal to tan 3 effective only for H t production in the context of a type I 2HDM model Here x A B Set these variables equal to the A scalar and B pseudoscalar coeffi cients of the tHb vertex in a type I 2HDM model Effective only for Ht production Process number that identifies the hard subprocess see tables 1 and 2 for valid entries Identifies the nature of the vector boson in associated Higgs production It corre sponds to variable IV of table 1 Identify the nature of the particles emerging from vector boson or top decays They correspond to variables IL and IL for x 1 2 respectively of tables 1 2 and 3 Valid entries are ALL and Wb Controls the type of top decay See sect 4 6 22 WITYPE PTVETO PARTn PDFGROUP PDFSET LAMBDAFIVE LAMBDAHERW SCHEMEOFPDF FPREFIX EVPREFIX EXEPREFIX NEVENTS MCMODE WGTTYPE RNDEVSEED BASES Valid entries are REMOVAL and SUBTRACTION Determines the definition of the Wt and Ht cross sections at the NLO See sect 4 4 Used in conjunction with FFACT and or FREN to set mass scales in Wt production See sect 4 4 The type of the incoming particle n with n 1 2 HW6 naming conven
5. collects the original codes written by the authors of the PDF fits as such for most of the densities it needs to read the files which contain the grids that initialize the PDF s These files which can also be downloaded from the MC NLO web page must either be copied into the running directory or defined in the running directory as logical links to the physical files by using 1n sn We stress that if the user runs MC NLO with the shell scripts the logical links will be created automatically at run time Starting from MC NLO version 3 4 the reading of parameters associated with the selected PDF set from LHAPDF has been made fully robust see sect 4 1 1 For this reason recent PDF sets are not being added to our PDF library and when adopting these and or when running HW LHAPDF must be used As stressed before consistent inputs must be given to the NLO and MC codes However in ref 1 we found that the dependence upon the PDFs used by HW6 is rather weak So one may want to run the NLO and HW6 adopting a regular NLL evolved set in the former case and the default HW6 set in the latter the advantage is that this option reduces the amount of running time of HW6 In order to do so the user must set the variable HERPDF equal to DEFAULT in the file MCatNLO inputs setting HERPDF EXTPDF will force HW6 to use the same PDF set as the NLO code On the other hand we found that the use of the HW default PDF set can be problematic in versions w
6. cross sections it displays an anomalously large event failure rate in the shower phase which is currently under investigation In what follows these two event generators will be referred to as HW6 and HW respectively and collectively as EvG Single top production in association with a charged Higgs has been added since version 3 4 including spin correlations which were not yet implemented in ref 16 The present version includes the upgrades of sub versions 3 41 and 3 42 For precise details of version changes see app A 1 A 9 1 1 Citation policy When using MC NLO with HW6 please cite ref 1 if HW is adopted please cite refs 1 24 In addition to ref 1 or to refs 1 24 if tt or bb events are generated please also cite ref 2 if s or t channel single top events are generated please also cite ref 25 if Wt single top events are generated please also cite ref 26 if H t single top events are generated please also cite ref 16 if W Z events are generated please also cite ref 27 The current user manual or any other user manuals relevant to past versions should not be cited unless the relevant papers mentioned above are cited too 1 2 Physics processes In the case of standard MC a hard kinematic configuration is generated on a event by event basis and it is subsequently showered and hadronized In the case of MCQ NLO all of the hard kinematic configurations are generated in advance and s
7. for the processes of table 2 spin correlations are obtained as described in ref 29 For these processes we shall denote in what follows the cross section obtained in a as the undecayed cross section and those obtained in b or c as the decayed cross sections We note that both for the processes in table 1 and for those in table 2 the This is the case for items b and c only if WGTTYPE 1 14 results of b and c are equal to the sum of the weights of all events stored in the event file possibly up to the contributions of those few events which HW6 is unable to shower and hadronize and which are therefore discarded with error messages in the MC run The situation is basically identical in the case of HW except for the fact that the result of b is to be found in the out file in the running directory rather than in the standard output and it is given in nb rather than in pb For the processes of table 2 the branching ratios used in the computation are deter mined by the values of the branching ratios for individual decay channels The following variables are relevant to top decays r jt iby at T S t udb BRTOPTOLEP T BRTOPTOHAD 4 1 with b and di any down type quark and antiquark respectively u an up type quark and l a charged lepton lepton and flavour universality are assumed In the case of W decays one has the analogous variables T r amp W di BRWTOLEP r W gt In BRWT
8. latter variable should coincide with the number of events in the event file The safety margin of 2 is amply sufficient to avoid oversampling The user may change such a safety margin by manipulating the MC executable script by hand by changing the N parameter passed there to HW 4 3 Inclusive NLO cross sections MC NLO integrates NLO matrix elements in order to produce the event file and thus computes as a by product the inclusive NLO cross section This cross section whose value is given in pb can be obtained from an MC NLO run in three different ways when running HW6 a It is printed out at the end of the NLO run search for Total for fully inclusive in the standard output b It is printed by HW6 at the end of the MC run search for CROSS SECTION PB in the standard output c It is equal to the integral of any differential distribution which covers the whole kinematically accessible range e g 0 lt pr lt 00 and on which no cuts are applied These three numbers are the same up to statistics which here means the number of generated events see the bottom of this section for further comments for the processes listed in table 1 For the processes listed in table 2 on the other hand the results of b and c are equal to that of a times the branching ratio s for the selected decay channel s times in the case of top decays other factors due to kinematic cuts specified in input see below This is so because
9. momentum preserved We have checked that this formal inconsistency has negligible actual conse quences The bug has been fixed in HW6 version 6 520 With older versions the fix may be found on the Fortran HERWIG wiki at http projects hepforge org fherwig trac report ticket 33 When this fix is implemented the statement PRESPL FALSE must be inserted in mcatnlo_hwdriver f at the place indicated by the comments therein e It has been found that a simpler form for the MC subtraction terms with respect to that of eq B 43 of ref 2 can be adopted this form is now implemented in version 3 4 This change is relevant only to QQ and single top production since for the other processes the new form and that of eq B 43 which is implemented in MCQ NLO version 3 3 or earlier coincide The differences between the two forms are equivalent to power suppressed terms This has been verified by comparing results obtained with version 3 4 29 for tt and single top s and t channel production at the Tevatron and the LHC and for bb production at the Tevatron with analogous results obtained with version 3 3 On the other hand bb production at the LHC does display large differences owing to the fact that the old form of MC subtraction terms has a pathology which affects this process Starting from version 3 4 bb production at the LHC may be considered safe Technical details on the new form of the MC subtraction terms will be posted on the MC
10. now generate this process either in the Standard Model or with a Lagrangian with anomalous i e non SM couplings according to ref 8 The most general amplitude for this process can be written as follows A Ao AgfAggz An Aggz A Az 4 9 with Ap the SM result An event weight i e the cross section will therefore be 2 2 2 Wror X Wo Ag wy Ax w2 7 w3 2Ag w4 2An4ws 27 w6 2Ag Ak w7 2Ag A ws 2AK4 Awg 4 10 The values of Ag Ak and A can be given in input using the script variables DELG1Z DELKAPZ and LAMANZ respectively By setting these three variables equal to zero one recovers the SM result A fourth script variable LAMFFAN corresponds to the quantity A introduced in eq 8 of ref 8 and serves the purpose of avoiding violations of unitarity if it is set equal to zero in input the program will re set it to the default value of 2 TeV Regardless of the values of Agi Ak and 7 given in input MC NLO can save the values of the weights w of eq 4 10 in the event file in order to do so the user must set CPLWGT YES in the inputs The quantities w change event by event and can be used to re weight the cross section and to obtain predictions simultaneously for any number We stress that this includes the SM case Ag 0 Ax 0 and 0 17 of combinations of anomalous couplings In the case of HW6 the information on w is available in the MC run through the comm
11. the prefix parameter The analogue of HWPPPATH for the ThePEG library Point to the base directory that holds the lib bin include and share directories This is typically the direc tory you have specified in the configure step of the ThePEG installation with the prefix parameter The physical address of the directory where the user wants to store the data and event files If left blank these files are stored in the running directory This variables must be set equal to a list of object files needed by the HW6 analy sis routines of the user for example HWUTI obj1 0 obj2 0 obj3 0 is a valid assignment This variables must be set equal to the name of the C analysis file relevant to HW runs for example HWPPANALYZER TopAnalysis is a valid assignment with TopAnalysis cc and TopAnalysis h being files in the HWppAnalyzer direc tory This variable must to be set equal to the name of the object file corresponding to the version of HW6 linked to the package for example HERWIGVER herwig6520 0 is a valid assignment The physical address of the directory where the PDF grids are stored Effective only if PDFLIBRARY THISLIB Set this variable equal to STATIC or DYNAMIC for linking with the static or dynamic LHAPDF library with HW6 This variable has no effect on how the HW executable is linked with LHAPDF Set this variable equal to the name of the directory where the local version of LHAPDF is
12. the run the program will crash or give meaningless results In addition to the above one will get the following files when running with HW FPREFIXMC run This file is produced by the HW executable and is internally needed for the run It is in simplified words the compiled FPREFIXMCinput file resulting from the Herwigt read FPREFIXMCinput command which is executed in the executable script FPREFIXMC tex Information including references describing the settings used in the HW run in TEX format By default all the files produced by the MCQ NLO are written in the running directory However if the variable SCRTCH to be set in MCatNLO inputs is not blank the data and event files will be written in the directory whose address is stored in SCRTCH such a directory is not created by the scripts and must already exist at run time 5 Script variables In the following we list all the variables appearing in MCatNLO inputs these can be changed by the user to suit his her needs This must be done by editing MCatNLO inputs For fuller details see the comments in MCatNLO inputs ECM The CM energy in GeV of the colliding particles FREN The ratio between the renormalization scale and a reference mass scale FFACT As FREN for the factorization scale HVQMASS The mass in GeV of the top quark except when IPROC 1 1705 when it is the mass of the bottom quark In this case HVQMASS must coincide with BMASS xMASS The mas
13. xIx x x x aie 7 cn a r x hho ww ax E 0m a v iy MW yen tx carom o 7 i Zax 2700 Ip o v H g HZ lily X 0 7 7 mmo ww x P80 r r mm Per E a E C0 x iW Table 1 Some of the processes implemented in MC NLO 4 0 see also table 2 H 2 represent nucleons or antinucleons IPROC 10000 generates the same processes as IPROC but eliminates the underlying event A void entry indicates that the corresponding variable is unused The Spin column indicates whether spin correlations in vector boson or top decays are included Vv neglected x or absent void entry when included spin correlations are obtained by direct integration of the relevant NLO matrix elements Spin correlations in Higgs decays to vector boson pairs e g H WtW I vl D are included in HW6 versions 6 520 and higher Processes 1705 and 2040 IC are not available for HW at present 1Ha gt Wt gt vi Z gt U X 1H gt Wt gt Z 3 X Table 2 Some of the processes implemented in MC NLO 4 0 see also table 1 Hi 2 represent nucleons or antinucleons For more details on Wt and H t production see sect 4 4 Spin correla tions for the processes in this table are implemented according to the method presented in ref 29 ba ba can either denote a b anti quark or a generic down type anti quark fa and f can denote a anti lepton or an anti quark See s
14. C 2850 and W Z IPROC 2870 2880 production can be given masses different from the pole masses These off shell effects are modeled by re weighting the cross section with skewed Breit Wigner functions in order to take into account the fact that by changing the invariant mass of the system produced one probes different values of Bjorken x s This re weighting is unitary i e it does not change the inclusive cross section For tt production the ranges of top and antitop masses are controlled by the parameters TiGAMMAX TiMASSINF and TiMASSSUP with i 1 2 for top and antitop respec tively For W W and W Z production one needs to use instead ViGAMMAX ViMASSINF and ViMASSSUP with i 1 2 for W and W7 and for W and Z respectively In both cases the mass ranges will be defined by formulae formally identical to those of eqs 4 6 and 4 7 In version 4 0 off shell effects are not implemented in the other processes in table 2 i e all channels of single top production Finally we point out that since spin correlations for the processes in table 2 are im plemented according to the method of ref 29 tree level matrix elements for leptonic final states are needed The codes for these have been generated with MadGraph MadEvent 35 36 and embedded into the MC NLO package When ILa 7 the corresponding particle is left undecayed by the NLO code and is passed as such to the MC code the information on spin correlations
15. C script We point out that if using HW6 the command runMC may be used with IPROC 1350 1L 1450 IL 1600 ID 1699 1705 1706 2000 2008 2600 ID 2699 2700 ID 2799 2800 2810 2815 2820 2825 to generate Z 7 W Higgs bb tt single top H W H Z and gt See below for comments on MCatNLO_rb inputs vector boson pair events with standard HW6 see the HW6 manual for more details Note that the events thus produced are weighted events except for single top production since we have set NOWGT FALSE in mcatnlo_hwdriver f however the user can freely change this setting For obvious reasons this does not work with HW In order to use standalone HW please consult the dedicated manual We stress that the input parameters are not solely related to physics masses CM energy and so on there are several of them which control other things such as the number of events generated These must also be set by the user according to his her needs see sect 5 Two such variables are HERWIGVER and HWUTI which were moved in version 2 0 from the Makefile to MCatNLO inputs and which are relevant to runs with HW6 The former variable must be set equal to the object file name of the version of HW6 currently adopted matching the one whose common blocks are included in the files mentioned in sect 3 The variable HWUTI must be set equal to the list of object files that the user needs in the analysis routines In the case of HW the analogue
16. IL are the analogue of HW6 1450 IL in HW6 either Wt or W can be produced whereas MC QNLO treats the two vector bosons separately For these processes as in HW6 IL 1 2 3 for li e u 7 but again the choice IL 0 is not allowed The lepton pair processes IPROC 1350 IL 1470 IL include spin correla tions when generating the angular distributions of the produced leptons However if spin correlations are not an issue the single vector boson production processes IPROC 1396 1397 1497 1498 can be used in which case the vector boson decay products are distributed by HW6 which then generates the decays according to phase space These processes should be considered only as a quicker alternative to lepton pair production with The same effect can be achieved by setting the HW6 parameter PRSOF 0 3Spin correlations between the decay products of vector boson pairs emerging from Higgs de cays were neglected in HW6 versions older than the current one 6 520 Please check the wiki at http projects hepforge org fherwig trac report for release reports on this and other improvements cmn 7 ah ln x omn ai n amam ne int E E E v x gt e A e E e ADM a aE wom amoa E ams _ e _ aime 7 7 x amoarx amore 7 _ xe r lt mmo orr r x mmo _ _ it Ca a E amA Coa 7 e e amwa E E A AA Cm r r lt mmm ou f o i e amma x x x x1 xX
17. OHAD r X W gt udi 3 4 2 Tw Tw Finally in the case of Z decays one has T Z 1 BRZTOEE aie 4 3 Tz The variables in eqs 4 1 4 3 can either be given a numerical value in input or computed at the LO in the SM by the code see sect 4 6 for details The numerical values of these variables are then combined to obtain the overall branching ratio for the decay channels selected which is done by setting the variables IL and TOPDECAY as explained in sect 4 6 see in particular table 3 For example for a top decaying into a W and any down type quarks with the W decaying to an electron muon or any quarks one sets ILg 6 TOPDECAY ALL and the resulting branching ratio will be 2 x BRTOPTOLEP 2 x BRTOPTOHAD As mentioned above in the case of top decays as opposed to hard W decays in Wt or WtW or W Z production the decayed cross section will include kinematic factors in addition to the branching ratios These factors are due to the fact that in general the range for the invariant mass of the pair of particles emerging from the W decay i e the virtuality of the W does not coincide with the maximum that is kinematically allowed For each top that decays the following kinematic factor will therefore be included in the decayed cross section Pte f f b qw inf qw sup PG ff 0m a with M dT t f fib T t gt ff b m M a ag TEZA 4 5 15 and qw inf qw sup the lower and upper limits of the W
18. Preprint typeset in JHEP style PAPER VERSION Cavendish HEP 10 12 CERN TH 2010 216 IPPP 10 62 DCPT 10 124 The MC NLO 4 0 Event Generator Stefano Frixione PH Department TH Unit CERN CH 1211 Geneva 28 Switzerland ITPP EPFL CH 1015 Lausanne Switzerland E mail Stefano Frixione cern ch Fabian Stoeckli PH Department CMG Group CERN CH 1211 Geneva 23 Switzerland E mail Fabian Stoeckli cern ch Paolo Torrielli ITPP EPFL CH 1015 Lausanne Switzerland E mail Paolo Torrielli epfl ch Bryan R Webber Cavendish Laboratory J J Thomson Avenue Cambridge CB3 OHE U K E mail webber hep phy cam ac uk Chris D White Department of Physics and Astronomy University of Glasgow Glasgow G12 8QQ Scotland U K Institute for Particle Physics Phenomenology Department of Physics Durham University Durham DH1 3LE U K E mail c d white durham ac uk ABSTRACT This is the user s manual of MCQ NLO 4 0 This package is a practical im plementation based upon the Fortran HERWIG and Herwig event generators of the MC NLO formalism which allows one to incorporate NLO QCD matrix elements con sistently into a parton shower framework Processes available in this version include the hadroproduction of single vector and Higgs bosons vector boson pairs heavy quark pairs single top single top in association with a W single top in association with a charged Higgs in type I or II 2HDM models lepton pairs and Higgs bosons in associa
19. QNLO web page A 9 From MC NLO version 3 4 to version 4 0 In this appendix we list the changes that occurred in the package from version 3 4 to subversions 3 41 and 3 42 to version 4 0 e A problem was found which affected top decays in the processes listed in table 2 except for H t not implemented in version 3 4 This implied that the identities of top decay products in n event samples of k events each could have been statistically not equivalent to those of one single event sample of n x k events for k 5000 or smaller Fixed in subversion 3 41 e All processes have been interfaced to HW except for H t production This im plies a new structure of the source directory and the addition of scripts MCatNLO_pp Script and a makefile Makefile_pp specific to HW e The linking to the static or dynamic LHAPDF library is now done via a shell variable MCatNLO_dyn Script and Makefile_dyn are thus obsolete and have been removed from the package e H t production has been implemented including spin correlations e Spin correlations and anomalous couplings have been included in W Z production e A numerical inaccuracy problem which affected the large rapidity large p region of leptons in W production has been fixed in subversion 3 42 In addition the convention on the range of lepton parton azimuthal angular differences has been changed from 0 7 to 7 7 in this process e The values of the fractions of the longit
20. The event generation step necessarily follows the integration step however for each integration step one can have an arbitrary number of event generation steps i e an arbi trary number of event files This is useful in the case in which the statistics accumulated with a given event file is not sufficient Suppose the user wants to create an event file editing MCatNLO inputs the user sets BASES ON to enable the integration step sets the parameter NEVENTS equal to the number of events wanted on tape and runs the code the information on the integration step unreadable to the user but needed by the code in the event generation step is written on files whose name begin with FPREFIX a string the user sets in MCatNLO inputs these files which we denote as data files have extensions data The name of the event file is EVPREFIX events where EVPREFIX is again a string set by the user Now suppose the user wants to create another event file to increase the statistics The user simply sets BASES OFF since the integration step is not necessary any longer however the data files must not be removed the information stored there is still used by the NLO code changes the string EVPREFIX failure to do so overwrites the existing event file while keeping FPREFIX at the same value as before and changes the value of RNDEVSEED the random number seed used in the event generation step failure to do so results in an event file identical to the previo
21. ad of gmake MCatNLO inputs HWPPInput inputs MCatNLO Script MCatNLO_pp Script Makefile Makefile_pp MCatNLO_rb inputs and the following subdirectories HW6Analyzer HWppAnalyzer srcCommon srcHerwig6 srcHerwigpp include The user will be primarily interested in MCatNLO inputs or MCatNLO_rb input see later which contains all input parameters common to HW6 and to HW Further input param eters specific to HW can be found in HWPPInput inputs The user will have to write his her own analysis routines and place them into the HW6Analyzer or the HWppAnalyzer directories for HW6 or HW runs respectively Sample analysis files are provided in or der to give the user a ready to run package The other files of the package must not be modified with the possible exception of the HW6 driver mcatnlo_hwdriver f and Les Houches interface mcatnlo_hwlhin f to be found in srcHerwig6 see sect 3 for further details When creating the executable our shell scripts determine the type of operating system and create a subdirectory of the source directory which we call the running directory whose name depends on the operating system For Linux the name of the running directory will be Linux when using HW6 and LinuxPP when using HW On other operating systems possible names of running directories are AlphaXX SunXX or RunXX with XX either the empty string or PP Tests on operating systems other than Linux have been performed sporad
22. alysis files mcatnlo_hwanzzz f e The linking to LHAPDF has been upgraded assuming the use of LHAPDF ver sion 5 0 or higher The file mcatnlo_lhauti f has been eliminated and replaced with mcatnlo_utilhav4 f which is however necessary only if the user wants to link with LHAPDF versions 4 xx in such a case the user will also need to edit the Makefile e The automatic assignment of Agcp when using LHAPDF is now to be considered ro bust This implies changes to mcatnlo_mlmtolha f the insertion of a dummy routine into mcatnlo_mlmtopdf f and mcatnlo_pdftomlm f and very minor changes to all main f files e Minor changes to mcatnlo_hbook f mainly affecting two dimensional plot outputs e A bug has been fixed which prevented one from choosing properly the W mass ranges in WtW production and subsequent decays in the case of ViGAMMAX lt 0 thanks to F Filthaut e A bug has been fixed which affected the computation of branching ratios in tt and single top production amp em q was previously called with argument mio rather than Miop This only affects event weights i e not distributions and is numerically very small e A bug in HERWIG versions 6 500 6 510 can lead to occasional violation of momen tum conservation when the HW6 parameter PRESPL FALSE hard subprocess rapidity preserved as is formally assumed in MC QNLO Therefore at present we leave this pa rameter at its default value PRESPL TRUE hard subprocess longitudinal
23. ber is printed out at the end of the NLO run search for Normalization factor due to decays in the standard output We conclude this section by stressing that while the result of a is always computed with a typical relative precision of 1074 those of b and c depend on the number of events generated Although it has been checked that upon increasing the number of events generated the results of b and c do approach that of a possibly times the branching ratios and kinematic factors option a is clearly preferred As mentioned above the decayed cross section of b or c can be obtained without any loss of accuracy by multiplying the undecayed cross section of a by the normalization factor printed out by the code at the end of the NLO run 4 4 Wt and H t production Owing to the interference with tt production which occurs in the gg and qq partonic channels starting at the NLO the Wt cross section is ill defined beyond the leading order in QCD One can still give an operative meaning to NLO Wt production but must always be aware of the potential biases introduced in this way This issue and its potential physics implications are discussed at length in ref 26 which the reader is strongly advised to consult before generating Wt events 16 Starting from MC NLO version 3 4 we have implemented two different definitions of the Wt cross section which we denoted by diagram removal and diagram subtrac tion in ref 26 The fo
24. e the local version of LHAPDF is installed This is typically the name of the directory where one finds the files 1ibLHAPDF a and 1ibLHAPDF so except for the final lib in the directory name As is well known a given PDF set has a preferred value of Agcop which should be used in the computation of short distance cross sections Upon setting LAMBDAFIVE in MCatNLO inputs equal to a negative value this choice is made automatically However when linking to PDFLIB or LHAPDF the code has to rely on the value Agcp stored by the PDF libraries in a common block This is far from ideal since Agcp is not a physical parameter and in particular is dependent upon the form adopted for as which may not be the same as that used in MCQ NLO Starting from version 3 4 the above automatic choice has been rendered more solid in the case of a linkage to LHAPDF the code now reads the value of as Mz i e of a physical quantity from the PDF library and converts it into a value for Agcp using the form of as Q used internally in MC NLO MC NLO will print out on the standard output when running the NLO code FPREFIXNLO 1og if using the scripts the value of Agcp used in the computation Such a value is now expected to be quite close to that listed under the column labeled with A MeV on our PDF library manual which can be found on the MCQ NLO web page Version 4 0 of MC NLO has been tested to link and run with several versions of LHAPDF In particular the us
25. ects 4 3 and 4 6 for fuller details Process 2040 IC is not available for HW at present a more limited physics content for this reason they have not been interfaced to HW There are a number of other differences between the lepton pair and single vector boson processes The latter do not feature the y Z interference terms Also their cross sections are fully inclusive in the final state fermions resulting from y Z or W The user can still select a definite decay mode using the HW6 variable MODBOS see sect 4 6 but the relevant branching ratio will not be included by MC NLO As stated previously these processes are not available for running with HW In NLO computations for single top production in the SM it is customary to distin guish between three production mechanisms conventionally denoted as the s channel t channel and Wt mode Starting from version 3 4 all three mechanisms are implemented in MC NLO s and t channel single top production correspond to setting IC 10 and IC 20 respectively For example according to tables 1 and 2 t channel single t events will be generated by entering IPROC 2021 These two channels can also be simulated simultaneously by setting IC 0 We point out that the Wt cross section is ill defined beyond the leading order in QCD which is also the case for H t production See sect 4 4 for more details In the case of vector boson pair production the process codes are the ne
26. er is not supposed to edit the Makefile s if linking with LHAPDF version 5 0 or higher If one is interested into linking with earlier versions of LHAPDF which is strongly deprecated then one must replace the string mcatnlo_uti o in the variable LUTIFILES in the Makefile with the string mcatnlo_utilhav4 o Again the version of LHAPDF used in the MC step when using HW is defined when installing HW We advise however to try to always use the same LHAPDF package i e link MC NLO with the same library as HW 4 1 2 PDF uncertainties The use of error sets to estimate the uncertainties due to PDFs which affect cross sections implies one computation for each of the members of the given error set when performing this procedure with MC QNLO we recommend to set HERPDF EXTPDF The procedure is straightforward but computing intensive and an approximate solution is that of reweighting the results obtained with the default PDF set by ratios of PDFs In order to do so information is necessary on the values of the fractional momenta of the incoming partons z and z2 and on the scale squared Q used in the computation of the PDFs Starting from MC NLO version 4 0 this information is made available to the user on an event by event basis without the necessity of kinematical reconstructions on the EvG event record In the case of HW6 the values of these quantities can be found in the required the use of the scripts in MCatNLO_dyn Script These sc
27. es interface mcatnlo_hwlhin f and the driver mcatnlo_hwdriver f have been upgraded e New script variables BRTOPTOx and BRWTOx with x LEP HAD yGAMMAX yMASSINF and yMASSSUP with y T1 T2 TOPDECAY WITYPE PTVETO have been introduced e The new script variables EXTRALIBS EXTRAPATHS and INCLUDEPATHS can be used to link to external libraries Their use has only been tested on a recent Scientific Linux release and they may be not portable to other systems 28 e The ranges of variables ILxCODE have been extended for several processes in order to account for the newly implemented hadronic decays e MCatNLO inputs and MCatNLO Script have been upgraded to reflect the changes above A new sample input file MCatNLOrb inputs is included which documents the use of an analysis producing plots in Root format Finally the possibility is given to link to a dynamic LHAPDF library through MCatNLO_dyn Script and Makefile_dyn e Front end Fortran routines rbook_fe f are provided to produce plots in Root format using the same syntax as for calling our HROOK type routines A companion C code is needed rbook_be cc These codes have been written by W Verkerke Examples of analysis routines using Root format have been added mcatnlo_hwangzzarb f A call to a release memory routine RCLOS has been added to mcatnlo_hwdriver f this is only needed when using a Root format output and a dummy body of RCLOS has been added to HBOOk format an
28. ess specific files mcatnlo_hgmain f mcatnlo_hgxsec f hgscblks h mcatnlo_hwanhgg f and a modi fication to mcatnlo_hwlhin f e Post 1999 PDF sets have been added to the MC NLO PDF library e Script variables have been added to MCatNLO inputs Most of them are only rele vant to Higgs production and don t affect processes implemented in version 2 0 One of them LAMBDAHERW may affect all processes in version 2 1 the variables LAMBDAFIVE and LAMBDAHERW are used to set the value of Agop in NLO and MC runs respectively whereas in version 2 0 LAMBDAFIVE controlled both The new setup is necessary since modern PDF sets have Agcp values which are too large to be supported by HERWIG Recall that the effect of using LAMBDAHERW different from LAMBDAFIVE is beyond NLO e The new script variable PDFPATH should be set equal to the name of the directory where the PDF grid files which can be downloaded from the MC NLO web page are stored At run time when executing runNLO or runMC or runMCatNLO logical links to these files will be created in the running directory in version 2 0 this operation had to be performed by the user manually e Minor bugs corrected in mcatnlo_hbook f and sample analysis routines A 3 From MC NLO version 2 1 to version 2 2 In this appendix we list the changes that occurred in the package from version 2 1 to version 222 e Single vector boson production has been added which implies new process specific files mcatn
29. gative of those adopted in MC NLO 1 0 for which the Les Houches interface was not yet available rather than those of standard HW6 Furthermore in the case of tt single t H W H Z W W and W Z production the value of IPROC alone may not be sufficient to fully determine the process type including decay products and variables IV IL and ILg are also needed see tables 1 and 2 In the case of top decays and of the decay of the hard W in Wt production the variables IL and IL have a more extended range of values than that of the variable IL which is relevant to lepton pair production and to which they are analogous notice however that in the latter case IL is not an independent variable and its value is included via IPROC In addition ILa 7 implies that spin correlations for the decay products of the corresponding particle are not taken into account as indicated in table 1 More details are given in sect 4 6 Apart from the above differences MCQ NLO and HW6 or HW behave in exactly the same way Thus the available user s analysis routines can be used in the case of MC NLO One should recall however that MC NLO always generates some events with negative weights see ref 1 therefore the correct distributions are obtained by summing weights with their signs i e the absolute values of the weights must NOT be used when filling the histograms With such a structure it is natural to create two separate execu
30. gives the total inclusive NLO cross section see sect 4 3 for more details Note that these weights are redefined by the EvG at MC run time according to its own convention see HW6 or HW manual The seed for the random number generation in the event generation step must be changed in order to obtain statistically equivalent but different event files Controls the integration step valid entries are ON and OFF At least one run with BASES ON must be performed see sect 4 2 E PDFLIBRARY HERPDF HWPATH HWPPPATH THEPEGPATH SCRTCH HWUTI HWPPANALYZER HERWIGVER PDFPATH LHALINK LHAPATH Valid entries are PDFLIB LHAPDF and THISLIB In the former two cases PDFLIB or LHAPDF is used to compute the parton densities whereas in the latter case the densities are obtained from our self contained PDF library If set to DEFAULT the EvG uses its internal PDF set controlled by NSTRU in the case of HW6 regardless of the densities adopted at the NLO level If set to EXTPDF the EvG uses the same PDFs as the NLO code see sect 4 1 The physical address of the directory where the user s preferred version of HW6 is stored The physical address of the directory where the user s preferred version of the HW tool is installed Point to the base directory that holds the lib bin include and share directories This is typically the directory one has specified in the configure step of the HW installation with
31. hep ph 0305252 12For vector boson pair production for historical reasons the different process codes 2800 2825 must be used 833 10 11 12 13 14 15 S Frixione Z Kunszt and A Signer Three jet cross sections to next to leading order Nucl Phys B 467 1996 399 arXiv hep ph 9512328 S Frixione A general approach to jet cross sections in QCD Nucl Phys B 507 1997 295 arXiv hep ph 9706545 B Mele P Nason and G Ridolfi QCD Radiative Corrections To Z Boson Pair Production In Hadronic Collisions Nucl Phys B 357 1991 409 S Frixione P Nason and G Ridolfi Strong corrections to W Z production at hadron colliders Nucl Phys B 383 1992 3 S Frixione A Next to leading order calculation of the cross section for the production of W W pairs in hadronic collisions Nucl Phys B 410 1993 280 L J Dixon Z Kunszt and A Signer Vector boson pair production in hadronic collisions at order a Lepton correlations and anomalous couplings Phys Rev D 60 1999 114037 arXiv hep ph 9907305 M L Mangano P Nason and G Ridolfi Heavy quark correlations in hadron collisions at next to leading order Nucl Phys B 373 1992 295 S Dawson Radiative Corrections To Higgs Boson Production Nucl Phys B 359 1991 283 A Djouadi M Spira and P M Zerwas Production of Higgs bosons in proton colliders QCD corrections P
32. here the default set is a so called LO PDF set We observed un expected features especially in pr spectra These features are manifest in both MCQNLO and HW standalone runs and disappear when other sets are used We did not inves tigate further the origin of the problem but we deprecate the use of the HW default PDF set with MCQ NLO and recommend using the same set as in the NLO run When using HW6 regardless of the PDFs used in the MC run users must delete the dummy PDFLIB routines PDFSET and STRUCTM from the HW6 source code as explained earlier 4 1 1 LHAPDF As mentioned above by setting PDFLIBRARY LHAPDF in the input file the code is linked to the LHAPDF library The user may choose whether to link to the static or to the dynamic LHAPDF library the latter will produce a smaller executable but otherwise results are identical to those obtained with the former This has obviously no effect for the MC step when using HW since the way LHAPDF is linked to the HW executable is defined in the HW installation Starting from MC NLO version 4 0 this choice is made Tn version 3 4 or earlier linking to the static library was the default and linking to the dynamic one 11 by assigning to the variable LHALINK in MCatNLO inputs the values STATIC or DYNAMIC respectively In order for the Makefile s to be able to find the LHAPDF library the variable LHAPATH in MCatNLO inputs should be set equal to the name of the directory wher
33. hys Lett B 264 1991 440 G Altarelli R K Ellis and G Martinelli Large Perturbative Corrections To The Drell Yan Process In QCD Nucl Phys B 157 1979 461 P Aurenche and J Lindfors QCD Corrections To Direct Lepton Production In Hadronic Collisions Nucl Phys B 185 1981 274 V Del Duca S Frixione and B R Webber MC NLO for Higgs Boson Production in preparation B W Harris E Laenen L Phaf Z Sullivan and S Weinzierl The fully differential single top quark cross section in next to leading order QCD Phys Rev D 66 2002 054024 arXiv hep ph 0207055 C Weydert et al Eur Phys J C 67 2010 617 arXiv 0912 3430 Unknown W T Giele S Keller and E Laenen QCD Corrections to W Boson plus Heavy Quark Production at the Tevatron Phys Lett B 372 1996 141 arXiv hep ph 9511449 G Marchesini B R Webber G Abbiendi I G Knowles M H Seymour and L Stanco HERWIG A Monte Carlo event generator for simulating hadron emission reactions with interfering gluons Version 5 1 April 1991 Comput Phys Commun 67 1992 465 G Corcella I G Knowles G Marchesini S Moretti K Odagiri P Richardson M H Seymour and B R Webber HERWIG 6 An event generator for hadron emission reactions with interfering gluons including supersymmetric processes JHEP 0101 2001 010 hep ph 0011363 G Corcella et al HERWIG 6 5 release note arXiv hep
34. ible the number of negative weight events We stress that the MC code will not change this number thus the tuning can and must be done only by running the NLO code The variables nitn control the integration step see sect 4 2 which can be skipped by setting nitn 0 If one needs to perform the integration step we suggest setting these variables as indicated in table 5 We now turn to the inputs for the MC executable presented in table 6 The variables whose names are in uppercase characters have been described in sect 5 The other variables are assigned by the shell script Their default values are given in table 7 The user can freely change the values of esctype and pdftype on the other hand the value of beammom must always be equal to half of the hadronic CM energy When LHAPDF is linked the value of PDFSET is sufficient to identify the parton density set In such a case PDFGROUP must be set in input equal to LHAPDF if the user wants to freeze the PDFs at the boundaries defined as the ranges in which the fits have been performed If one chooses to extrapolate the PDFs across the boundaries one should set PDFGROUP LHAEXT in input 32 EVPREFIX events event file NEVENTS pdftype PART1 PART2 beammom beammom number of events 0 gt Herwig PDFs 1 otherwise hadron types beam momenta IPROC 2850 60 70 80 W W ZZ ZW ZW PDFGROUP PDF group 1 PDFSET PDF id number 1 PDFGROUP
35. ically on Mac s and in the case of HW6 only we recommend using Linux whenever possible The running directory contains all the object files and executable files and in general all the files produced by the MC NLO while running It must also contain the relevant grid files see sect 4 1 or links to them if the library of parton densities provided with the MC NLO package is used In the subdirectory HW6Analyzer of the source directory the user will find the files mcatnlo_hwanzzz f which use a version of HBOOK written by M Mangano that outputs plots in TopDrawer format and mcatnlo_hwangzzz_rb f which use front end Fortran rou tines written by W Verkerke 30 for filling histograms in Root format These are sample HW6 analysis routines one for each of the processes implemented in this package They are provided here to give the user a ready to run package but they should be replaced with ap propriate codes according to the user s needs Examples of how to use these analysis files in MC NLO are given in the otherwise identical MCatNLO inputs and MCatNLO_rb inputs files see sect 4 for more details on input cards In the subdirectory HWppAnalyzer the user will find analogous codes to be used when running with HW We do not provide sample analyses for all processes in this case and only the TopDrawer format version is given since Root is a native C code that can be much more easily used with HW than with HW6 In addition t
36. ies In this document we briefly describe how to run the MC NLO package implemented according to the formalism introduced in ref 1 The production processes now available are listed in tables 1 and 2 The process codes IPROC and the variables IV and IL will be explained below Hj represent hadrons in practice nucleons or antinucleons The information given in refs 1 2 allows the implementation in MC NLO of any production process provided that the formalism of refs 3 4 is used for the computation of cross sections to NLO accuracy The production matrix elements have been taken from the fol lowing references vector boson pairs 5 6 7 WZ production and decay with anomalous couplings 8 heavy quark pairs 9 Standard Model Higgs 10 11 single vector boson 12 lepton pairs 13 associated Higgs 14 single top s and t channel 15 single top in asso ciation with a charged Higgs 16 those for single top production in association with a W have been re derived and thoroughly compared to those of ref 17 This documentation refers to MCQ NLO version 4 0 The major difference with respect to previous MC NLO versions is that in the present one all processes can be simulated either with Fortran HERWIG 18 19 20 or with Herwig 21 22 with the exception in the latter case of single top in association with a charged Higgs and of bb production While the latter process is fully implemented at the level of short distance
37. installed See sect 4 1 1 This has no effect on which version of LHAPDF is used in the MC step when running HW 94 LHAOFL Set LHAOFL FREEZE to freeze PDFs from LHAPDF at the boundaries or equal to EXTRAPOLATE otherwise See LHAPDF manual for details EXTRALIBS Set this variable equal to the names of the libraries which need be linked LHAPDF is a special case and must not be included in this list EXTRAPATHS Set this variable equal to the names of the directories where the libraries which need be linked are installed INCLUDEPATHS Set this variable equal to the names of the directories which contain header files possibly needed by C files provided by the user also in HW6 runs via HWUTI Acknowledgments It is a pleasure to thank the co authors of the MCQ NLO papers E Laenen P Motylinski and P Nason for having contributed so much to many different aspects of the MC QNLO project and for stimulating discussions We thank W Verkerke for having provided us with a Fortran interface to C Root calling routines We thank F Filthaut for having uncovered and fixed a bug in version 3 4 B R W thanks the CERN theory group for frequent hospitality The work of P T is supported by the Swiss National Science Foun dation C D W is supported by the STFC Postdoctoral Fellowship Collider Physics at the LHC Finally we are indebted with all the members of experimental collaborations unfortunately too numerous to be exp
38. is lost However the user can still force particular decay modes during the MC run When the chosen EvG is HW6 in the case of vector bosons one proceeds in the same way as in standard HW6 using the MODBOS variables see sect 3 4 of ref 19 However top decays cannot be forced in this way because the decay is treated as a three body process the W boson entry in HEPEVT is for information only Instead the top branching ratios can be altered using the HWMODK subroutine see sect 7 of ref 19 This is done separately for the t and t For example CALL HWMODK 6 1 D0 100 12 11 5 0 0 forces the decay t v e b while leaving t decays unaffected Note that the order of the decay products is important for the decay matrix element NME 100 to be applied correctly The relevant statements should be inserted in the HERWIG main program corresponding to mcatnlo_hwdriver f in this package after the statement CALL HWUINC and before the loop over events A separate run with CALL HWMODK 6 1 D0 100 12 11 5 0 0 should be performed if one wishes to symmetrize the forcing of t and t decays since calls to HWMODK from within the event loop do not produce the desired result On the other hand when HW is used the relevant decay channels may be selected with suitable set instructions that may be conveniently included in the HWPPInput inputs file In the case a specific decay channel should be switched on off a set Herwig Particles command
39. licitly mentioned here who used the code and gave us precious suggestions and feedback Appendices A Version changes A 1 From MC NLO version 1 0 to version 2 0 In this appendix we list the changes that occurred in the package from version 1 0 to version 2 0 e The Les Houches generic user process interface has been adopted e As a result the convention for process codes has been changed MC NLO process codes IPROC are negative e The code mcatnlo_hwhvvj f which was specific to vector boson pair production in version 1 0 has been replaced by mcatnlo_hwlhin f which reads the event file according to the Les Houches prescription and works for all the production processes implemented e The Makefile need not be edited since the variables HERWIGVER and HWUTI have been moved to MCatNLO inputs where they must be set by the user e A code mcatnlo_hbook f has been added to the list of utility codes It contains a simplified version written by M Mangano of HBOOK and it is only used by the sample analysis routines mcatnlo_hwangzzz f As such the user will not need it when linking to a self contained analysis code 95 We also remind the reader that the HW6 version must be 6 5 or higher since the Les Houches interface is used A 2 From MC NLO version 2 0 to version 2 1 In this appendix we list the changes that occurred in the package from version 2 0 to version 2 1 e Higgs production has been added which implies new proc
40. lo_hwdriver f for HW6 or by the scripts MCatNLO_pp Script for HW Therefore if these files are not employed when running the EvG s the user will be responsible for switching the underlying event off manually if so desired For processes with a SM Higgs denoted by H in the final state the user may specify the identities of its decay products when using HW6 by adding ID to the process code The conventions for ID are the same as in HW6 namely ID 1 6 for uu tt 7 8 9 te wtp Tr 10 11 for WtW ZZ and 12 for yy Furthermore ID 0 gives quarks of all flavours and ID 99 gives all decays It should be stressed that the event for e file does not contain the Higgs decay products and therefore is independent of the value of ID the decay is dealt with by HW6 In the case of HW the user will instead select the decay products using the HW input file HWPPInput inputs which we shall describe later This means that ID has no effect in SM Higgs production when using HW Process codes IPROC 1360 IL and 1370 IL do not have an analogue in HW6 they are the same as 1350 IL except for the fact that only a Z or a y respectively is exchanged The value of IL determines the lepton identities and the same convention as in HW6 is adopted IL 1 6 for l e Ve H Vu T Vr respectively see also table 3 At variance with HW6 IL cannot be set equal to zero Process codes IPROC 1460 IL and 1470
41. lo_sbmain f mcatnlo_sbxsec f svbcblks h mcatnlo_hwansvb f and a modification to mcatnlo_hwlhin f e The script variables WWIDTH and ZWIDTH have been added to MCatNLO inputs These denote the physical widths of the W and Z bosons used to generate the mass distributions of the vector bosons according to the Breit Wigner function in the case of single vector boson production vector boson pair production is still implemented only in the zero width approximation A 4 From MC NLO version 2 2 to version 2 3 In this appendix we list the changes that occurred in the package from version 2 2 to version 2 3 e Lepton pair production has been added which implies new process specific files mcatnlo_llmain f mcatnlo_llxsec f llpcblks h mcatnlo_hwanllp f and modifi cations to mcatnlo_hwlhin f and mcatnlo_hwdriver f 26 e The script variable AEMRUN has been added since the computation of single vector boson and lepton pair cross sections is performed in the MS scheme the on shell scheme was previously used for single vector boson production e The script variables FRENMC and FFACTMC have been eliminated e The structure of pseudo random number generation in heavy flavour production has been changed to avoid a correlation that affected the azimuthal angle distribution for the products of the hard partonic subprocesses e A few minor bugs have been corrected which affected the rapidity of the vector bosons in single vector boso
42. ly in conjunction with that EvG If the shell scripts are not used to run the codes the inputs are given to the NLO or MC codes during an interactive talk to phase the complete sets of inputs for our codes are reported in app B 2 for vector boson pair production 4 1 Parton densities Since knowledge of the parton densities PDFs is necessary in order to get the physical cross section a PDF library must be linked For the NLO runs the possibility exists to link the now obsolete CERNLIB PDF library PDFLIB or its replacement LHAPDF 32 however we also provide a self contained PDF library with this package which is faster than PDFLIB and contains PDF sets released after the last and final PDFLIB version 8 04 most of these sets are now included in LHAPDF The three PDF libraries mentioned 10 above can also be used for HW6 runs As far as HW is concerned the only possibility is that of linking to LHAPDF or that of using the default HW PDF set If this is desired the variable HERPDF must be set equal to DEFAULT see below for further comments A complete list of the PDFs available in our PDF library can be downloaded from the MC NLO web page The user may link one of the three PDF libraries all that is necessary is to set the variable PDFLIBRARY in the file MCatNLO inputs equal to THISLIB if one wants to link to our PDF library and equal to PDFLIB or to LHAPDF if one wants to link to PDFLIB or to LHAPDF Our PDF library
43. n production a 2 3 effect and the assignment of Agop for the LO and NLO PDF sets of Alekhin A 5 From MC NLO version 2 3 to version 3 1 In this appendix we list the changes that occurred in the package from version 2 3 to version 3 1 e Associated Higgs production has been added which implies new process specific files mcatnlo_vhmain f mcatnlo_vhxsec f vhgcblks h mcatnlo_hwanvhg f and modifi cations to mcatnlo_hwlhin f and mcatnlo_hwdriver f e Spin correlations in WtW production and leptonic decay have been added the relevant codes mcatnlo_vpmain f mcatnlo_vhxsec f have been modified the sample analysis routines mcatnlo_hwanvbp f have also been changed Tree level matrix elements have been computed with MadGraph MadEvent 35 36 which uses HELAS 37 the relevant routines and common blocks are included in mcatnlo_helas2 f and MEcoupl inc e The format of the event file has changed in several respects the most relevant of which is that the four momenta are now given as pz Py pz m up to version 2 3 we had Px Py Pz E Event files generated with version 2 3 or lower must not be used with version 3 1 or higher the code will prevent the user from doing so e The script variables GAMMAX MASSINF and MASSSUP have been replaced with xGAMMAX xMASSINF and xMASSSUP with x H V1 V2 e New script variables IVCODE IL1CODE and IL2CODE have been introduced e Minor changes have been made to the routines that put the pa
44. n the case of dilepton and of H Z production the Z may decay into charged lepton or neutrino pairs The identities of the decay products are determined by setting IL for dilepton production or IL for HZ production according to the values given in table 3 In the case of W Z production only Z decays into charged lepton pairs are implemented with the identities of the leptons determined by setting the variable IL equal to 1 3 or 5 see table 3 For the processes in table 2 it is also possible to force the code to use the LO values of the relevant leptonic and hadronic branching ratios by entering negative values for the top W and Z widths variables TWIDTH WWIDTH and ZWIDTH in MCatNLO inputs In such a case the values of BRTOPTOLEP BRTOPTOHAD BRWTOLEP BRWIOHAD BRZTOEE given in the input file will be ignored and replaced by their LO values which are equal to 1 9 for top and W leptonic decays and equal to 1 3 for top and W hadronic decays Likewise the top W and Z widths will be computed using the LO SM formulae Spin correlations are implemented in the processes in table 2 according to the method of ref 29 which is based on a zero width approximation for the decaying particles Neverthe Tn other words hadronic Z decays may be simulated by the EvG but spin correlations are not included in such cases 19 less the top quark and antiquark in tt production IPROC 1706 and the vector bosons in WtW IPRO
45. o the files listed above the user will need a version of the EvG code s to be used HW6 18 19 20 or HW 21 22 As stressed in ref 1 for MC NLO we do not modify the existing LL shower algorithm However since MCQNLO versions 2 0 and higher make use of the Les Houches interface first implemented in HERWIG 6 5 the version of HW6 must be 6 500 or higher When using HW6 on most operating systems users will need to delete the dummy subroutines UPEVNT UPINIT PDFSET and STRUCTM from the standard HW6 package to permit linkage of the corresponding routines from the MCQ NLO package As a general rule the user is strongly advised to use the most recent versions of HERWIG which were the ones used in the testing phase of MCQNLO 3 Prior to running 3 1 Usage with HW6 When using HW6 the user must be aware of the fact that the files mcatnlo_hwdriver f mcatnlo_hwlhin f which can be found in srcHerwig6 and the files mcatnlo_hwanzez f mcatnlo_hwanzez_rb f which can be found in HW6Analyzer contain the statement INCLUDE HERWIG65 INC which indicates that the code will link to version 6 500 or higher for the reasons explained above In the current MC NLO release the file HERWIG65 INC contains the statement INCLUDE herwig6520 inc We do not assume that the user will adopt version 6 520 which is the latest release for this reason the user may need to edit the file HERWIG65 INC and change the statement above into INCLUDE
46. of HWUTI is HWPPANALYZER which must be set equal to the name of the C analysis code the user means to use The variable HERWIGVER does not have an analogue when using HW its role is played by the variables HWPPPATH and THEPEGPATH which must be set equal to the physical address of the base directories of the HW and of the ThePEG installations respectively The sample input file MCatNLO inputs provided in this package is relevant to tt pro duction and subsequent t and t leptonic decays Similar sample inputs are given in the file MCatNLO_rb inputs which is identical to the former except that at the end of the MC run an output file in Root format will be produced as opposed to the output file in TopDrawer format produced by MCatNLO inputs for this to happen the user will have to edit MCatNLO_rb inputs in order to insert the path to the Root libraries for the machine on which the run is performed shell variables EXTRAPATHS and INCLUDEPATHS We stress that apart from the differences in the output formats MCatNLO inputs and MCatNLO_rb inputs have exactly the same meaning Thus although for the sake of brevity we shall often re fer only to MCatNLO inputs in this manual all the issues concerning the inputs apply to MCatNLO_rb inputs as well Furthermore as stressed above an explicit example of how to obtain results in Root format is only necessary in the case of HW6 runs and therefore MCatNLO_rb inputs provided here should be used on
47. on block DOUBLE PRECISION WGTACP 10 COMMON CWGTACP WGTACP with WGTACP I 1 w wror In the case of HW the information is available in the event file right before the lt event gt tag in the following form an_cplwgt wo Wror Wo Wror 4 6 Decays MC NLO is intended primarily for the study of NLO corrections to production cross sections and distributions NLO corrections to the decays of produced particles are not included As for spin correlations the situation in version 4 0 is summarized in tables 1 and 2 they are included for all processes except ZZ production For the latter processes quantities sensitive to the polarization of produced particles are not given correctly even to leading order For such quantities it may be preferable to use the standard HERWIG MCs which do include leading order spin correlations Following HW6 conventions spin correlations in single vector boson processes are au tomatically included using the process codes IPROC relevant to lepton pair production in other words if one is interested in including spin correlations in e g W production and subsequent decays into ptv one needs to use IPROC 1461 rather than IPROC 1497 and MODBOS 1 3 In order to avoid an unnecessary proliferation of IPROC values this strategy has not been adopted in other cases tt single t H9W H Z WtW WEZ in which spin correlations are included if the variables IL and IL the latter is used
48. ph 0210213 M Bahr et al Herwig Physics and Manual Eur Phys J C 58 2008 639 arXiv 0803 0883 hep ph 34 22 23 24 25 26 30 31 32 33 34 35 36 37 M Bahr et al Herwig 2 3 Release Note arXiv 0812 0529 hep ph P Torrielli and S Frixione Matching NLO QCD computations with PYTHIA using MC NLO JHEP 1004 2010 110 arXiv 1002 4293 Unknown S Frixione F Stoeckli P Torrielli B R Webber NLO QCD corrections in Herwig with MC NLO preprint Cavendish HEP 10 14 CERN TH 2010 212 S Frixione E Laenen P Motylinski and B R Webber Single top production in MC NLO JHEP 0603 2006 092 arXiv hep ph 0512250 S Frixione E Laenen P Motylinski B R Webber and C D White Single top hadroproduction in association with a W boson JHEP 0807 2008 029 arXiv 0805 3067 hep ph S Frixione A Oh Anomalous couplings in WZ hadroproduction in preparation E Boos et al Generic user process interface for event generators arXiv hep ph 0109068 S Frixione E Laenen P Motylinski and B R Webber Angular correlations of lepton pairs from vector boson and top quark decays JHEP 0704 2007 081 arXiv hep ph 0702198 W Verkerke Nikhef unpublished L Lonnblad ThePEG Pythia7 herwig and Ariadne Nucl Instrum Meth A 559 2006 246 Web page http www thep lu se ThePEG M R Whalle
49. r o mcatnlo_pdftomlm o mcatnlo_libofpdf o dummies o HWUTI HERWIGVER MC with PDFLIB mcatnlo_hwdriver o mcatnlo_hwlhin o mcatnlo_hwanvbp o mcatnlo_hbook o mcatnlo_str o mcatnlo_mlmtopdf o dummies oHWUTI HERWIGVER CERNLIB MC with LHAPDF mcatnlo_hwdriver o mcatnlo_hwlhin o mcatnlo_hwanvbp o mcatnlo_hbook o mcatnlo_str o mcatnlo_mlmtolha o dummies oHWUTI HERWIGVER LHAPDF The process specific codes mcatnlo_vbmain o and mcatnlo_vbxsec o for the NLO exe cutable and mcatnlo_hwanvbp o the HW6 analysis routines in the MC executable need to be replaced by their analogues for other production processes The variable SYSFILE must be set either equal to alpha o or to Linux o or to sun o according to the architecture of the machine on which the run is performed For any other architecture the user should provide a file corresponding to alpha f etc which he she will easily obtain by modifying alpha f The variables HWUTI and HERWIGVER have been described in sect 5 In order to create the object files eventually linked static compilation is always recommended for example g77 Wall fno automatic on Linux B 2 The input files Here we describe the inputs to be given to the NLO and MC executables in the case of vector boson pair production The case of other production processes is completely analogous When the shell scripts are used to run the MCQ NLO two files are created FPREFIXNLOinput and FPREFIXMCinput which are read by
50. ripts are now obsolete and have been removed from the package with their companion Makefile_dyn 12 common block DOUBLE PRECISION UX1 UX2 UQ2 COMMON CPDFRWGT UX1 UX2 UQ2 with UX1 2 UX2 a2 and UQ2 Q in GeV In the case of HW the information can be found right after the compulsory event information in the form pdf x1 z2 Q We point out that reweighting with different PDFs is never exact in the context of an NLO computation and this is especially true when such computation is interfaced to an event generator as in MC NLO since PDF effects in Sudakovs cannot possibly be taken into account in this way We therefore recommend performing PDF reweighting with utmost care 4 2 Event file The NLO code creates the event file In order to do so it goes through two steps first it integrates the cross sections integration step and then using the information gathered in the integration step produces a set of hard events event generation step Integration and event generation are performed with a modified version of the SPRING BASES package 33 We stress that the events stored in the event file contain only the partons involved in the hard subprocesses Owing to the modified subtraction introduced in the MCQ NLO formalism see ref 1 they do not correspond to pure NLO configurations and should not be used to plot physical observables Parton level observables must be reconstructed using the fully showered events
51. rmer computation is carried out by setting WITYPE REMOVAL in MCatNLO inputs while the latter corresponds to WITYPE SUBTRACTION A practical appli cation of these ideas to a phenomenological analysis is presented in ref 34 In Wt production the factorization renormalization scale is assigned the value of the variable PTVETO whose units are GeV if FFACT lt 0 FREN lt 0 This option should be used for testing purposes only it is not recommended in the generation of event samples for experimental studies In MC NLO version 4 0 we have implemented Ht production in a generic 2HDM model see ref 16 When m lt m this process interferes with tt production and the same considerations as for Wt production apply here In order to avoid the proliferation of input variables WITYPE also controls which definition of the H t cross section is used When m gt m there is no interference with tt production and therefore H t production is well defined and does not require any special treatment at the level of matrix elements In this mass range the user must set WITYPE REMOVAL to run the code Finally we point out that there are four input variables that are specific to H t production TYPEIORIT TANBETA ACPL and BCPL see sect 5 for more details 4 5 W Z production and anomalous couplings Starting from MC NLO version 4 0 spin correlations have been added to W Z produc tion Furthermore the user can
52. rsion 3 2 to version 3 3 e Spin correlations have been added to tt and single t production processes which imply modifications to several codes mcatnlo_qqmain f mcatnlo_qqxsec f mcatnlo_stmain f mcatnlo_stxsec f mcatnlo_hwlhin f and mcatnlo_hwdriver f Tree level matrix ele ments have been computed with MadGraph MadEvent 35 36 e The matching between NLO matrix elements and parton shower is now smoother in Higgs production which helps eliminate one unphysical feature in the pr spectra of the accompanying jets The code mcatnlo_hgmain f has been modified Technical details on this matching procedure will be posted on the MC NLO web page e The new script variable TWIDTH has been introduced e All instances of HWWARN s i n have been replaced with HWWARN s i in HW6 related codes This is consistent with the definition of HWWARN in HW6 versions 6 510 and higher the user must be careful if linking to HW6 versions in which the former form of HWWARN is used Although HW6 6 510 compiles with g95 or gfortran MCQ NLO 3 3 does not A 8 From MC NLO version 3 3 to version 3 4 In this appendix we list the changes that occurred in the package from version 3 3 to version 3 4 e Wt production has been implemented which implies new process specific codes mcatnlo_wtmain_dr f mcatnlo_wtmain_ds f mcatnlo_wtxsec_dr f and mcatnlo_wtxsec_ds f e Owing to the implementation of Wt production and of top hadronic decays the Les Houch
53. rtons on the HW6 mass shell for lepton pair heavy quark and vector boson pair production effects are beyond the fourth digit e The default electroweak parameters have been changed for vector boson pair pro duction in order to make them consistent with those used in other processes The cross sections are generally smaller in version 3 1 wrt previous versions the dominant effect being the value of sin 0w we have now sin 0w 0 2311 in lower versions sin Oy 1 my m3 The cross sections are inversely proportional to sint Ow A 6 From MC NLO version 3 1 to version 3 2 In this appendix we list the changes that occurred in the package from version 3 1 to version 3 2 27 e Single t production has been added which implies new process specific files mcatnlo_stmain f mcatnlo_stxsec f stpcblks h mcatnlo_hwanstp f and modifi cations to mcatnlo_hwlhin f and mcatnlo_hwdriver f e LHAPDF library is now supported which implies modifications to all main f files and two new utility codes mcatnlo_lhauti f and mcatnlo_mlmtolha f e New script variables Vud LHAPATH and LHAOFL have been introduced e A bug affecting Higgs production has been fixed which implies a modification to mcatnlo_hgxsec f Cross sections change with respect to version 3 1 only if FFACTH 1 by O 1 in the range 1 2 lt FFACT lt 2 A 7 From MC NLO version 3 2 to version 3 3 In this appendix we list the changes that occurred in the package from ve
54. s in GeV of the particle x with x HGG W Z U D S C B G 2 xWIDTH BRTOPTOx BRWTOx BRZTOEE IBORNHGG xGAMMAX xMASSINF xMASSSUP Vud AEMRUN TYPEIORII TANBETA xCPL IPROC IVCODE ILxCODE TOPDECAY The physical Breit Wigner width in GeV of the particle x with x HGG W Z T for H WF Z and t respectively Branching ratio for top decay channels 2 t gt lnb when x LEP and t gt ud b when x HAD Lepton and flavour universality is assumed Branching ratio for W decay channels W I when x LEP and W ud when x HAD Lepton and flavour universality is assumed Branching ratio for Z decay channels Z ll Lepton universality is assumed Valid entries are 1 and 2 If set to 1 the exact top mass dependence is retained at the Born level in Higgs production If set to 2 the m oo limit is used If xGAMMAX gt 0 controls the width of the mass range for Higgs x H vector bosons x V1 V2 and top x T1 T2 the range is MASS GAMMAX x WIDTH Off shell effects for top are only implemented in tt production Lower limit of the Higgs x H vector boson x V1 V2 and top x T1 T2 mass range used only when xGAMMAX lt 0 Upper limit of the Higgs x H vector boson x V1 V2 and top x T1 T2 mass range used only when xGAMMAX lt 0 CKM matrix elements with u U C T and d D S B Set VUD VUS VUB 0 to use values of PDG2003 Set it to YES to use running
55. s that the options Reconstruction0ption General and InitialInitialBoostOption LongTransBoost must be used when running HW These are automatically set by our script and the user must not change them The latter option is presently available only in the trunk version of HW 2 4 2 4 Running It is straightforward to run MC NLO First edit MCatNLO inputs and write there all the input parameters for the complete list of the input parameters see sect 5 Further parameters specific to HW that control the behaviour of this EvG can be set in HWPPInput inputs As the last line of the file MCatNLO inputs write runMCatNLO Finally execute MCatNLO inputs from the bash shell This procedure will create the NLO and MC executables and run them using the inputs given in MCatNLO inputs which guar antees that the parameters used in the NLO and MC runs are consistent Should the user only need to create the executables without running them or to run the NLO or the MC only he she should replace the call to runMCatNLO in the last line of MCatNLO inputs by calls to compileNLO compileMC runNLO runMC which have obvious meanings Take note that in the case of HW the compileMC com mand actually does not compile the HW executable This has already been done in the installation of the HW package The compileMC command rather compiles the chosen analyzer creates all necessary soft links in the running directory and creates the executable M
56. should be added e g set t gt b bbar c On0ff Off to switch off the decay of top quarks into b b c For more details please consult the HW manual 4 7 Results As in the case of standard HERWIG the form of the results will be determined by the user s analysis routines However in addition to any files written by the user s analysis routines the MC NLO writes the following files 20 FPREFIXNLOinput the input file for the NLO executable created according to the set of input parameters defined in MCatNLO inputs where the user also sets the string FPREFIX See table 4 FPREFIXNLO 1og the log file relevant to the NLO run FPREFIXxxx data xxx can assume several different values These are the data files created by the NLO code They can be removed only if no further event generation step is foreseen with the current choice of parameters FPREFIXMCinput analogous to FPREFIXNLOinput but for the MC executable See ta ble 6 FPREFIXMC 1log analogous to FPREFIXNLO 1log but for the MC run FPREFIXMC out produced only in HW runs Contains the result for the inclusive NLO rate see sect 4 3 EVPREFIX events the event file where EVPREFIX is the string set by the user in MCatNLO inputs EVPREFIXxxx events xxx can assume several different values These files are temporary event files which are used by the NLO code and eventually removed by the shell scripts They MUST NOT be removed by the user during
57. single t produc tion See ref 29 lOBRWTOLEP is also used in WWT production W hadronic decays are not implemented in this process hence the branching ratio is only used as a rescaling factor for event weights 18 the definitions of these variables o 4 Q no decay Table 3 Decays of the W s produced in the hard process or originating from top antitop decays and of the Z s The symbol q denotes all hadronic W decays Values different from 1 2 or 3 for W decays are only allowed in tt and single top production all channels In the case of top antitop decays it is also possible to generate events in which the top decays into a W and any down type quark hence the notations by and ba in table 2 The identity of the latter is determined according to the CKM matrix values For this to happen one needs to set TOPDECAY ALL in MCatNLO inputs If on the other hand one wants to always generate t Wb decays one needs to set TOPDECAY Wb in such a case event weights and thus the decayed cross section as defined in sect 4 3 will be multiplied by a factor V2 V2 V2 V2 In MC NLO version 4 0 spin correlations in leptonic processes involving intermediate Z bosons are always included except in the case of ZZ production These processes are dilepton production IPROC 1350 IL H Z production IPROC 2700 ID W Z production IPROC 2870 and W Z production IPROC 2880 I
58. tables which we improperly denote as NLO and MC The former has the sole scope of creating the event file the latter is just the EvG executable In the case of HW6 this file coincides with the actual HW6 executable In the case of HW it is a script that calls the relevant HW executables 2 Structure of the package 2 1 Working environment We have written shell scripts and Makefile s which will simplify the use of the package considerably In order to use them the computing system must support bash shell and gmake Should they be unavailable on the user s computing system the compilation and running of MC NLO requires more detailed instructions in this case we refer the reader to app B This appendix will serve also as a reference for a more advanced use of the package 2 2 Source and running directories The package can be downloaded as a tarball from the web page http www hep phy cam ac uk theory webber MCatNLO The structure of the directories that contain the source codes has become more involved starting from MC NLO version 4 0 in order to deal with the possibility of interfacing to more than one EvG The directory tree will be created automatically when unpacking the tarball We describe it here briefly The directory in which the tarball will be unpacked will be called the source directory The source directory contains the following files For Macs running under OSX v10 or higher make can be used inste
59. the NLO and MC executable respectively We start by considering the inputs for the NLO executable presented in table 4 The variables whose name is in uppercase characters have been described in sect 5 The other variables are assigned by the shell script Their default values are given in table 5 Users who run the package without the script should use the values given in table 5 The variable zi controls to a certain extent the number of negative weight events generated by the MCQ NLO see ref 1 Therefore the user may want to tune this g FPREFIX prefix for BASES files EVPREF IX prefix for event files ECM FFACT FREN FFACTMC FRENMC energy scalefactors IPROC 2850 60 70 80 WW ZZ ZW ZW WMASS ZMASS UMASS DMASS SMASS CMASS BMASS GMASS PART1 PART2 PDFGROUP PDFSET M_W M_Z quark and gluon masses hadron types PDF group and id number LAMBDAFIVE Lambda_5 lt 0 for default SCHEMEOFPDF scheme NEVENTS number of events WGTTYPE 10 gt wgt 4 1 1 1 gt wet w w RNDEVSEED seed for rnd numbers zi zi nitn nitno itmx1 itmx2 Table 4 Sample input file for the NLO code for vector boson pair production FPREFIX and EVPREFIX must be understood with SCRTCH in front see sect 5 Variable Default value Zi 0 2 nitn 10 0 BASES ON OFF Table 5 Default values for script generated variables in FPREFIXNLOinput parameter in order to reduce as much as poss
60. tion with a W or Z Spin correlations are included for all processes except ZZ production This document is self contained but we emphasise the main differences with respect to previous versions Work supported in part by the UK Science and Technology Facilities Council and by the Swiss National Science Foundation tOn leave of absence from INFN Sez di Genova Italy Contents 1 Generalities 1 1 Citation policy 1 2 Physics processes Structure of the package 2 1 Working environment 2 2 Source and running directories Prior to running 3 1 Usage with HW6 3 2 Usage with HW Running 4 1 Parton densities 4 1 1 LHAPDF 4 1 2 PDF uncertainties 4 2 Event file 4 3 Inclusive NLO cross sections 4 4 Wt and At production 4 5 WZ production and anomalous couplings 4 6 Decays 4 7 Results Script variables Version changes A 1 From MC NLO version 1 0 to version 2 0 A 2 From MC NLO version 2 0 to version 2 1 A 3 From MC NLO version 2 1 to version 2 2 A 4 From MC NLO version 2 2 to version 2 3 A 5 From MC NLO version 2 3 to version 3 1 A 6 From MC NLO version 3 1 to version 3 2 A 7 From MC NLO version 3 2 to version 3 3 A 8 From MC NLO version 3 3 to version 3 4 A 9 From MC NLO version 3 4 to version 4 0 Running the package without the shell scripts B 1 Creating the executables B 2 The input files 10 11 12 13 14 16 17 18 20 21 25 25 26 26 26 27 27 28 28 30 30 30 31 1 Generalit
61. tions are used P PBAR N NBAR The name of the group fitting the parton densities used the labeling conventions of PDFLIB are adopted Unused when linked to LHAPDF The number of the parton density set according to PFDLIB conventions the pair PDFGROUP PDFSET identifies the densities for a given particle type When linked to LHAPDF use the numbering conventions of LHAGLUE 32 The value of Agop for five flavours and in the MS scheme used in the computation of NLO cross sections A negative entry sets Agcp equal to that associated with the PDF set being used The value of Agcp used in MC runs this parameter has the same meaning as Agcp in HERWIG The subtraction scheme in which the parton densities are defined Our integration routine creates files with name beginning by the string FPREFIX Most of these files are not directly accessed by the user See sects 4 2 and 4 7 The name of the event file begins with this string See sects 4 2 and 4 7 The names of the NLO and MC executables begin with this string this is useful in the case of simultaneous runs The number of events stored in the event file eventually processed by the EvG See sect 4 2 for comments relevant to HW Valid entries are HW6 and HWPP for using HW6 or HW in the MC run respectively Valid entries are 0 and 1 When set to 0 the weights in the event file are 1 When set to 1 they are w with w a constant such that the sum of the weights
62. tored in a file which we call the event file see sect 4 2 the event file is then read by the EvG which showers The matching to PYTHIA has been worked out analytically in ref 23 for the case of initial state radiation The production processes implemented in computer codes are still limited in number and are not part of the present package and hadronizes each hard configuration Since version 2 0 the events are handled by the Les Houches generic user process interface 28 see ref 2 for more details Therefore in MC NLO the reading of a hard configuration from the event file is equivalent to the generation of such a configuration in a standard MC The signal to the EvG that configurations should be read from an event file using the Les Houches interface is a negative value of the process code IPROC this accounts for the negative values in tables 1 and 2 In the case of heavy quark pair Higgs Higgs in association with a W or Z and lepton pair through Z y exchange production the codes are simply the negative of those for the corresponding standard HW6 MC processes Where possible this convention will be adopted for additional MC NLO processes re gardless of the EvG actually used to shower events Consistently with what happens in standard HW6 by subtracting 10000 from IPROC one generates the same processes as in tables 1 and 2 but eliminates the underlying event Note that this operation is per formed within mcatn
63. udinal momenta of the incoming partons 71 and 29 and that of the mass scale squared Q in GeV used in the computations of the PDFs are now stored in the event file B Running the package without the shell scripts In this appendix we describe the actions that the user needs to take in order to run the package without using the shell scripts and the Makefile Examples are given for vector boson pair production but only trivial modifications are necessary in order to treat other production processes B 1 Creating the executables An MC NLO run requires the creation of two executables for the NLO and MC codes respectively The files to link depend on whether one uses PDFLIB LHAPDF or the PDF library provided with this package we list them below 30 NLO with private PDFs mcatnlo_vbmain o mcatnlo_vbxsec o mcatnlo_helas2 o mcatnlo_date o mcatnlo_int o mcatnlo_uxdate o mcatnlo_uti o mcatnlo_str o mcatnlo_pdftomlm o mcatnlo_libofpdf o dummies o SYSFILE NLO with PDFLIB mcatnlo_vbmain o mcatnlo_vbxsec o mcatnlo_helas2 o0 mcatnlo_date o mcatnlo_int o mcatnlo_uxdate o mcatnlo_uti o mcatnlo_str o mcatnlo_mlmtopdf o dummies o SYSFILE CERNLIB NLO with LHAPDF mcatnlo_vbmain o mcatnlo_vbxsec o mcatnlo_helas2 o0 mcatnlo_date o mcatnlo_int o mcatnlo_uxdate o mcatnlo_lhauti o mcatnlo_str o mcatnlo_mlmtolha o dummies o SYSFILE LHAPDF MC with private PDFs mcatnlo_hwdriver o mcatnlo_hwlhin o mcatnlo_hwanvbp o mcatnlo_hbook o mcatnlo_st
64. us one the number NEVENTS generated may or may not be equal to the one chosen in generating the former event file s We point out that data and event files may be very large If the user wants to store them in a scratch area this can be done by setting the script variable SCRTCH equal to the 13 physical address of the scratch area see sect 4 7 For historical reasons the formats of the event files to be used by HW6 and HW are not identical the former uses an internal MC NLO format whereas the latter uses the Les Houches format It is crucial to realize that event files meant to be showered by say HW must not be showered by HW6 and the other way round This has nothing to do with the format of the event files which one may consider changing and is instead due to the fact that the short distance cross sections in MC NLO do depend on the EvG used for the shower It has to be noted that in the case of HW the command runMC will typically not process the entire event file The reason for this is the following By using the Les Houches interface a run will not stop automatically when reaching the end of the event file which will then be oversampled This happens since an EvG counts as events processed only those which are successfully showered which are typically slightly less than those read from the event file In order to avoid oversampling we therefore give in input to HW a number of events equal to 98 times NEVENTS the
65. virtuality which can be chosen in input In particular if V1GAMMAX gt 0 one will have dw inf WMASS ViGAMMAX x WWIDTH dw sup WMASS ViGAMMAX x WWIDTH 4 6 On the other hand if V1GAMMAX lt 0 one has qw inf VIMASSINF qw sup VIMASSSUP 4 7 The ranges in eqs 4 6 or 4 7 apply to the W emerging from the decay of the top quark in tt production and of the top or antitop in single top production all channels The corresponding ranges for the W emerging from the decay of the antitop quark in tt production are identical to those above except for the replacement of V1 with V2 The user is also allowed to generate events by fixing the virtuality of the W emerging from top antitop decays equal to the W pole mass by setting xGAMMAX 0 with x V1 V2 In such a case the decayed cross section will be equal to the undecayed cross section times the branching ratios times a factor dI t f f b day v a 2 Fw My 4 8 for each decaying top quark The decayed cross section will have therefore to be interpreted as differential in the W virtuality squared doubly differential in the case of tt production and will be expressed in pb GeV or pb GeV for tt production units The branching ratios and kinematics factors for each decaying particles are multiplied to give a single number always less than or equal to one which is by definition the ratio of the decayed over the undecayed cross section This num
66. y D Bourilkov and R C Group The Les Houches accord PDFs LHAPDF and LHAGLUE arXiv hep ph 0508110 S Kawabata A New version of the multidimensional integration and event generation package BASES SPRING Comput Phys Commun 88 1995 309 C D White S Frixione E Laenen and F Maltoni Isolating Wt production at the LHC JHEP 0911 2009 074 arXiv 0908 0631 hep ph T Stelzer and W F Long Automatic generation of tree level helicity amplitudes Comput Phys Commun 81 1994 357 arXiv hep ph 9401258 F Maltoni and T Stelzer MadEvent Automatic event generation with MadGraph JHEP 0302 2003 027 arXiv hep ph 0208156 H Murayama I Watanabe and K Hagiwara HELAS HELicity amplitude subroutines for Feynman diagram evaluations KEK 91 11 35

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