The MC@NLO 3.3 Event Generator
Contents
1. Table 4 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 4 The negative sign of IPROC tells HERWIG to use Les Houches interface routines We now turn to the inputs for the MC executable presented in table 4 The variables whose names are in uppercase characters have been described in sect 4 The other variables are assigned by the shell script Their default values are given in table 5 The user can Variable Default value esctype 0 pdftype 0 1 HERPDF DEFAULT EXTPDF beammom EMC 2 Table 5 Default values for script generated variables in MCinput 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 19 In the case of y Z W5 Higgs or heavy quark production the MC executable can be run with the2. 11 A Version changes 14 A 1 From MC NLO version 1 0 to version 2 0 14 A 2 From MC NLO version 2 0 to version 2 1 14 A 3 From MC NLO version 2 1 to version 2 2 15 A 4 From MC NLO version 2 2 to version 2 3 15 A 5 From MC NLO version 2 3 to version 3 1 15 A 6 From MC NLO version 3 1 to version 3 2 16 A 7 From MC NLO version 3 2 to version 3 3 16 B Running the package without the shell scripts 17 B 1 Creating the executables 17 B 2 The input files 18 1 Generalities In this documentation file we briefly describe how to run the MCQ NLO package imple mented according to the formalism introduced in ref 1 When using MC NLO please cite refs 1 2 if single top events are generated please also cite ref 3 The production processes now available are listed in table 1 The process codes IPROC will be explained below H 2 represent hadrons in practice p or p The treatment of undecayed vec tor boson pair production within MC NLO has been described in ref 1 that of heavy quark pair production in ref 2 The NLO matrix elements for these processes have been taken from refs 4 5 6 7 The information given in refs 1 2 allows the implementa tion in MC NLO of any production process provided that the formalism of refs 8 9 is used for the computation of cross sections to NLO accuracy For specific information on single top production see ref 3 The matrix elements for Standard Model Higgs single vector boson lep
3. app B 2 for vector boson pair production 3 1 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 29 We stress that the events stored in the event file just contain the partons involved in the hard suprocesses Owing to the modified subtraction introduced in the MC 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 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 fil
4. corresponding positive input process codes IPROC 1350 1399 1499 1600 ID 1705 1706 2000 2008 2600 ID or 2700 ID to generate a standard HERWIG run for comparison purposes Then the input event file will not be read instead parton configurations will be generated by HERWIG 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 hep ph 0305252 3 S Frixione E Laenen P Motylinski and B R Webber Single top production in MC NLO JHEP 0603 2006 092 arXiv hep ph 0512250 4 B Mele P Nason and G Ridolfi QCD Radiative Corrections To Z Boson Pair Production In Hadronic Collisions Nucl Phys B 357 1991 409 5 S Frixione P Nason and G Ridolfi Strong corrections to W Z production at hadron colliders Nucl Phys B 383 1992 3 6 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 7 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 8 S Frixione Z Kunszt and A Signer Three jet cross sections to next to leading
5. easily obtain by modifying alpha f The variables HWUTI and HERWIGVER have been described in sect 4 In order to create the object files eventually linked static compilation is always recommended for example g77 Wall fno automatic on Linux S47 B 2 The input files In this appendix 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 the NLO and MC executable respectively We start by considering the inputs for the NLO executable presented in table 2 The variables whose name is in uppercase characters have been described in FPREFIX prefix for BASES files EVPREFIX prefix for event files ECM FFACT FREN FFACTMC FRENMC energy scalefactors IPROC 2850 60 70 80 WW ZZ ZW ZW WMASS ZMASS M_W M_Z UMASS DMASS SMASS CMASS BMASS GMASS quark and gluon masses PART1 PART2 PDFGROUP PDFSET hadron types PDF group and id number LAMBDAFIVE Lambda_5 lt 0 for default SCHEMEOFPDF scheme NEVENTS number of events WGTTYPE 0 gt wgt 1 1 1 gt wet w w RNDEVSEED seed for rnd numbers zi zi nitn nitng itmx1 itmx2 Table 2 Sample input file for the NLO code for vector boson pair production FPREFIX and EVPREFIX must
6. must be set equal to a list of object files needed by the analysis routines of the user for example HWUTI obj1 0 obj2 0 obj3 0 is a valid assignment This variable must to be set equal to the name of the object file corresponding to the version of HERWIG linked to the package for example HERWIGVER herwig65 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 the name of the directory where the local version of LHAPDPF is installed Set LHAOFL FREEZE to freeze PDFs from LHAPDF at the boundaries or equal to EXTRAPOLATE otherwise See LHAPDF manual for details Acknowledgement Many thanks to P Nason for contributions to the heavy quark code and valuable discussions on all aspects of the MCQ NLO project and to E Laenen and P Motylinski for the work done on single t and tt production We also thank V Drollinger and B Quayle for testing a preliminary version of the WtW code with spin correlations and Y Fang and B Mellado for testing a preliminary version of the Higgs code in version 3 3 We are grateful for the hospitality of CERN Theory Division where most of this work was carried out 13 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 inte
7. order Nucl Phys B 467 1996 399 arXiv hep ph 9512328 9 S Frixione A general approach to jet cross sections in QCD Nucl Phys B 507 1997 295 arXiv hep ph 9706545 10 S Dawson Radiative Corrections To Higgs Boson Production Nucl Phys B 359 1991 283 11 A Djouadi M Spira and P M Zerwas Production of Higgs bosons in proton colliders QCD corrections Phys Lett B 264 1991 440 12 G Altarelli R K Ellis and G Martinelli Large Perturbative Corrections To The Drell Yan Process In QCD Nucl Phys B 157 1979 461 13 P Aurenche and J Lindfors QCD Corrections To Direct Lepton Production In Hadronic Collisions Nucl Phys B 185 1981 274 14 C Oleari V Del Duca S Frixione and B R Webber MC NLO for Higgs Boson Production in preparation 15 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 For vector boson pair production for historical reasons the different process codes 2800 2825 must be used 20 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 Frixione and B R Webber The MC NLO event generator arXiv hep ph 0207182 Frixione and B R Webber The MC NLO 2 0 event generator arXiv hep ph 0307146
8. process type including decay products and new variables IV IL and IL have been introduced see table 1 The variables IL and IL can take the same values as IL relevant to lepton pair production notice however that in the latter case IL is not an independent variable and its value is included via IPROC in addition IL 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 3 2 Apart from the above differences MC NLO and HERWIG 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 refs 1 2 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 executables which we improperly denote as NLO and MC The former has the sole scope of creating the event file the latter is just HERWIG augmented by the capability of reading the event file 1 2 Package files The package consists of the following files e Shell utilities MCatNLO Script MCatNLO inputs Makefile e Utility codes MEcoupl inc alpha f dummies f linux f mcatnlo_date f mcatnlo_hbook f mcatnlo_helas2 f mcatnlo_hwdum
9. 30 31 e The matching between NLO matrix elements and parton shower is now smoother in Higgs production see ref 23 which helps eliminate one unphysical feature in the pr spectra of the accompanying jets The code mcatnlo_hgmain f has been modified 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 HERWIG related codes This is consistent with the definition of HWWARN in HERWIG ver sions 6 510 and higher the user must be careful if linking to HERWIG versions in which the former form of HWWARN is used Although HERWIG 6 510 compiles with g95 or gfortran MC NLO 3 3 does not 16 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 e 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_libo
10. 7Z X Table 1 Processes implemented in MC NLO 3 3 H denotes the Standard Model Higgs boson and the value of ID controls its decay as described in the HERWIG manual and below The values of IV IL IL and IL control the identities of vector bosons and leptons as described below In single t production the value of IC controls the production processes as described below 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 v neglected x or absent void entry Spin correlations in Higgs decays are included by HERWIG e g in H WtW I vI D 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 according to phase space 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 defi
11. Frixione and B R Webber The MC NLO 2 2 event generator arXiv hep ph 0309186 Frixione and B R Webber The MC NLO 3 1 event generator arXiv hep ph 0506182 S S S S Frixione and B R Webber The MCQ NLO 2 3 event generator arXiv hep ph 0402116 S S Frixione and B R Webber The MC NLO 3 2 event generator arXiv hep ph 0601192 S Frixione E Laenen P Motylinski and B R Webber Angular correlations of lepton pairs from vector boson and top quark decays in MCQNLO in preparation S Frixione and B R Webber Non constant p veto in MCQNLO implementation of Higgs production in preparation E Boos et al Generic user process interface for event generators arXiv hep ph 0109068 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 ph 0210213 M R Whalley D Bourilkov and R C Group The Les Houches accord PDFs LHAPDF and LHAG
12. LUE 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 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 21
13. NLO 1log 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 4 FPREFIXMC 1log analogous to FPREFIXNLO 1log but for the MC run 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 the run the program will crash or give meaningless results By default all the files produced by the MCQNLO 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 A 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 of the colliding particles FREN The ratio b
14. Preprint typeset in JHEP style PAPER VERSION Cavendish HEP 06 28 GEF TH 19 2006 The MC NLO 3 3 Event Generator Stefano Frixione INFN Sezione di Genova Via Dodecaneso 33 16146 Genova Italy E mail Stefano Frixione cern ch Bryan R Webber Cavendish Laboratory J J Thomson Avenue Cambridge CB3 0HE U K E mail webber hep phy cam ac uk ABSTRACT This is the user s manual of MC NLO 3 3 This package is a practical imple mentation based upon the HERWIG event generator of the MCQ NLO formalism which allows one to incorporate NLO QCD matrix elements consistently into a parton shower framework Processes available in this version include the hadroproduction of single vec tor and Higgs bosons vector boson pairs heavy quark pairs single top lepton pairs and Higgs bosons in association with a W or Z Spin correlations are included for all processes except ZZ and WZ production This document is self contained but we emphasise the main differences with respect to previous versions KEYWORDS QCD Monte Carlo NLO Computations Resummation Hadronic Colliders Work supported in part by the UK Particle Physics and Astronomy Research Council Contents 1 Generalities 1 1 1 Mode of operation 2 1 2 Package files 5 1 3 Working environment 6 1 4 Source and running directories 7 2 Prior to running 7 2 1 Parton densities 7 3 Running 3 1 Event file 9 3 2 Decays 10 3 3 Results 10 4 Script variables
15. be also 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 MCQNLO with the shell scripts the logical links will be created automatically at run time 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 the MC is rather weak So one may want to run the NLO and MC adopting a regular NLL evolved set in the former case and the default HERWIG set in the latter the advantage is that this option reduces the amount of running time of the MC 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 the MC to use the same PDF set as the NLO code Regardless of the PDF s used in the MC run users must delete the dummy PDFLIB routines PDFSET and STRUCTM from HERWIG as explained earlier 3 Running It is straightforward to run the MCQ NLO First edit MCatNLO inputs and write there all the input parameters for the complete list of the input parameters see sect 4 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 paramete
16. be understood with SCRTCH in front see sect 4 sect 4 The other variables are assigned by the shell script Their default values are given in table 3 Users who run the package without the script should use the values given Variable Default value Zi 0 2 nitn 10 0 BASES ON OFF Table 3 Default values for script generated variables in FPREFIXNLOinput in table 3 The variable zi controls to a certain extent the number of negative weight events generated by the MC NLO see ref 1 Therefore the user may want to tune this parameter in order to reduce as much as possible 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 18 see sect 3 1 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 3 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 WW ZZ ZW ZW PDFGROUP PDF group 1 PDFSET PDF id number 1 PDFGROUP 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
17. ches interface is a negative value of the process code IPROC this accounts for the negative values in table 1 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 HERWIG MC processes Where possible this convention will be adopted for additional MCQ QNLO processes Consistently with what happens in standard HERWIG by subtracting 10000 from IPROC one generates the same processes as in table 1 but eliminates the underlying event Higgs decays are controlled in the same way as in HERWIG that is by adding ID to the process code The conventions for ID are the same as in HERWIG namely ID 1 6 for ua tt 7 9 for ete 7 77 10 11 for W W ZZ and 12 for yy Furthermore ID 0 gives quarks of all flavours and ID 99 gives all decays Process codes IPROC 1360 IL and 1370 IL do not have an analogue in HERWIG 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 HERWIG is adopted IL 1 6 for lj e Ve H Vu T Vy respectively At variance with HERWIG IL cannot be set equal to zero Process codes IPROC 1460 IL and 1470 IL are the analogue of HERWIG 1450 IL in HERWIG either Wt or W can be produced The same effect can be achieved by s
18. e the type of operating system and create a subdirectory of the source directory which we call the running directory whose name is Alpha Sun Linux or Darwin depending on the operating system If the operating system is not known by our scripts the name of the working directory is Run 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 2 1 or links to them if the library of parton densities provided with the MCQ NLO package is used 2 Prior to running Before running the code the user needs to edit the following files mcatnlo_hwanzaez f mcatnlo_hwdriver f mcatnlo_hwlhin f We do not assume that the user will adopt the latest release of HERWIG although as ex plained above it must be version 6 500 or higher For this reason the files mcatnlo_hwdriver f and mcatnlo_hwlhin f must be edited in order to modify the INCLUDE HERWIGXX INC command to correspond to the version of HERWIG the user is going to adopt mcatnlo_hwdriver f contains a set of read statements which are necessary for the MC to get the input pa rameters see sect 3 for the input procedure these read statements must not be modi fied or eliminated Also mcatnlo_hwdriver f calls the HERWIG routines which perform showering hadronization decays see sect 3 2 for more details on this issue and so forth the user can freel
19. ept ZZ and W Z production For the latter processes quantities sensitive to the polarisation of produced particles are not given correctly even to leading order For such quantities it may be preferable to use the standard HERWIG MC which does include leading order spin correlations Following HERWIG conventions spin correlations in single vector boson processes are automatically included using the process codes IPROC relevant to lepton pair production In order to avoid an unnecessary proliferation of IPROC values this strategy has not been adopted in other cases tt single t H W H Z WWT in which spin correlations are included if the variables IL and IL the latter is used only in tt and WtW production are given values ranging 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 The value of IL determines the identity of the leptons emerging from the decay and the same convention as in HERWIG is adopted see the HERWIG manual and sect 1 1 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 is lost However the user can still force particular decay modes during the MC run In the case of vector bosons one proceeds in the same way as in standard HERWIG using the MODBOS variables see sect 3 4 of ref 26 However top decays cannot be forced in this way because the decay
20. er that identifies the hard subprocess see table 1 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 leptons emerging from vector boson or top decays x 1 2 They correspond to variables IL and IL of table 1 The type of the incoming particle n with n 1 2 HERWIG naming conventions 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 28 The value of Agep for five flavours and in the MS scheme used in the computation of NLO cross sections 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 These files are not directly accessed by the user for more details see sect 3 1 The name of the event file begins with this string for more details see sect 3 1 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 even
21. es 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 Agcp 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 14 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 2 2 e Single vector boson production has been added which implies new process specific files mcatnlo_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 Brei
22. es whose name begin with FPREFIX a string the user sets in MCatNLO inputs these files which we denotes 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 previous 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 physical address of the scratch area see sect 3 3 3 2 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 3 3 is summarized in table 1 they are included for all processes exc
23. etting the HERWIG parameter PRSOF 0 whereas MC NLO treats the two vector bosons separately HERWIG IL 1 2 3 for l e u 7 but again the choice IL 0 is not allowed For these processes as in IPROC IV IL ILg Spin Process 1350 IL V HH gt Z gt X 1360 IL V HH gt ZS X 1370 IL V HH gt 7 Sin X 1460 IL vV HH gt Wt a lio X 1470 IL v HH W gt a X 1396 x MMe gt X fifi X 1397 x HWH Z X 1497 x HH gt Wt xX 1498 x HH gt W 4 X 1600 ID HH gt H X 1705 H H gt bb X 1706 ame ee x HH gt tt X 1706 i j vV HH gt t gt o v t gt bl gt 0 X 2000 I 7 x HH gt t t xX 2000 IC i v HiH t gt blF vi t gt bl pi X 2001 IC 7 x HH gt t xX 2001 IC i v HiH gt t gt bl H X 2004 IC 7 x HH gt t xX 2004 IC i v HiH t blt vi X 2600 ID 1 7 x HH gt H Wt X 2600 ID 1 i v HH Wt gt lt vni X 2600 ID 1 7 x HiH HoW 4 X 2600 ID 1 i vV HH gt H W gt n X 2700 ID 0 7 x HH gt H Z X 2700 ID 0 i J HH gt H Z 3 X 2850 Tose x HH gt WtW 4 X 2850 i j vV mH gt Wt gt u W gt P X 2860 7 7 x HiH gt 29794 X 2870 7 7 x HH gt Wt Z94 X 2880 7 7 x HH W
24. etween 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 mass in GeV of the particle x with x HGG W Z U D S C B G xWIDTH The physical Breit Wigner width in GeV of the particle x with x HGG W Z T for H W Z and t respectively IBORNHGG 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 xGAMMAX If xGAMMAX gt 0 controls the width of the mass range for Higgs x H and vector bosons x V1 V2 the range is MASS GAMMAX x WIDTH xMASSINF Lower limit of the Higgs x H or vector boson x V1 V2 mass range used only when xGAMMAX lt 0 xMASSSUP Vud AEMRUN IPROC IVCODE ILxCODE PARTn PDFGROUP PDFSET LAMBDAFIVE LAMBDAHERW SCHEMEOFPDF FPREFIX EVPREFIX EXEPREFIX NEVENTS WGTTYPE Upper limit of the Higgs x H or vector boson x V1 V2 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 Vud is only used in single t production Set it to YES to use running Qem in lepton pair and single vector boson production set it to NO to use Qem 1 137 0359895 Process numb
25. fpdf o dummies o SYSFILE e 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 e 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 e MC with private PDFs mcatnlo_hwdriver o mcatnlo_hwlhin o mcatnlo_hwanvbp o mcatnlo_hbook o mcatnlo_str o mcatnlo_pdftomlm o mcatnlo_libofpdf o dummies o HWUTI HERWIGVER e 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 e MC with LHAPDF mcatnlo_hwdriver o mcatnlo_hwlhin o mcatnlo_hwanvbp o mcatnlo_hbook o mcatnlo_str o mcatnlo_mlmtolha o dummies o HWUTI HERWIGVER LHAPDF The process specific codes mcatnlo_vbmain o and mcatnlo_vbxsec o for the NLO exe cutable and mcatnlo_hwanvbp o the HERWIG analysis routines in the MC executable need to be replaced by their analogues for other production processes which can be easily read from the list given in sect 1 2 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
26. is treated as a three body process the W boson entry in HEPEVT is for informa tion only Instead the top branching ratios can be altered using the HWMODK subroutine see sect 7 of ref 26 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 gt eb 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 3 3 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 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 3Non factorizable spin correlations of virtual origin are not included in W W_ tt and single t produc tion See ref 22 10 See table 2 FPREFIX
27. lly smaller in version 3 1 wrt previous versions the dominant effect being the value of sin Ow we have now sin Oy 0 2311 in lower versions sin Oy 1 m mz The cross sections are inversely proportional to sinf Oy 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 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 modifications 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 FFACTA 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 version 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
28. 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 30 31 which uses HELAS 32 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 15 Px Py Pz 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 partons on the HERWIG 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 genera
29. my f mcatnlo_int f mcatnlo lhauti f mcatnlo_libofpdf mcatnlo mlmtolha mcatnlo_mlmtopdf Fh Fh Fh Fh mcatnlo_pdftomlim mcatnlo_str f mcatnlo_uti f mcatnlo_uxdate c sun f trapfpe c General HERWIG routines mcatnlo_hwdriver f mcatnlo_hwlhin f Process specific codes mcatnlo_hwanbtm mcatnlo_hwanhgg mcatnlo_hwanllp mcatnlo_hwantop mcatnlo_hwanstp mcatnlo_hwansvb mcatnlo_hwanvbp Hh Fh Fh Fh Fh Fh Fh sFh mcatnlo_hwanvhg mcatnlo_hgmain f mcatnlo_hgxsec f mcatnlo_llmain mcatnlo_llxsec mcatnlo_qqmain mcatnlo_qqxsec mcatnlo_sbmain mcatnlo_sbxsec mcatnlo_stmain mcatnlo_stxsec mcatnlo vbmain mcatnlo vbxsec mcatnlo vhmain Fh Fh FH Fh Fh Fh Fh Fh Hi Ph Fi EF mcatnlo_vhxsec hgscblks h hvqcblks llpcblks stpcblks svbcblks vhgcblks rrpppy pe These files can be downloaded from the web page http www hep phy cam ac uk theory webber MCatNLO The files mcatnlo_hwangzz f which appear in the list of the process specific codes are sample HERWIG analysis routines They are provided here to give the user a ready to run package but they should be replaced with appropriate codes according to the user s needs In addition to the files listed above the user will need a version of the HERWIG code 25 26 27 As stressed in ref 1 for the MCQ NLO we do not modify the existing LL shower algorithm However since MC NLO versions 2 0 and higher make use of the Les Houches interface fi
30. nite decay mode using the variable MODBOS see sect 3 2 but the relevant branching ratio will not be included automatically by MC QNLO In the case of 7 production the branching ratios are C q 20 3 qi being the electric charge in units of the positron charge of the fermion 7 selected through MODBOS and C 1 for leptons and 3 for quarks Notice that 20 3 Ciq the sum including all leptons and quarks except the top Thus the total rate predicted by MC NLO in the case of lepton pair production can also be recovered by multiplying the corresponding single vector boson total rate by the relevant branching ratio In NLO computations for single top production it is customary to distinguish between three production mechanisms conventionally denoted as s channel t channel and Wt mode In version 3 3 only the former two are implemented The user can generate them either separately by setting IC 10 and IC 20 for s and t channel production respectively or together by setting IC 0 For example according to table 1 t channel single t events will be generated by entering IPROC 2021 In the case of vector boson pair production the process codes are the negative of those adopted in MC NLO 1 0 for which the Les Houches interface was not yet available rather than those of standard HERWIG Furthermore in the case of tt single t H W H Z and WtW production the value of IPROC alone may not be sufficient to fully determine the
31. rface 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 hwanzzz f As such the user will not need it when linking to a self contained analysis code We also remind the reader that the HERWIG 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 process specific files mcatnlo_hgmain f mcatnlo_hgxsec f hgscblks h mcatnlo_hwanhgg f and a modification tomcatnlo_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 process
32. rs 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 We point out that the command runMC may be used with IPROC 1350 IL 1450 IL 1600 ID 1699 1705 1706 2000 2008 2600 ID 2699 2700 ID 2799 to generate Z 7 W Higgs bb tt single top H W or H Z events with standard HERWIG see the HERWIG manual for more details We stress that the input parameters are not solely related to physics masses CM energy and so on there are a few 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 4 Two such variables are HERWIGVER and HWUTI which were moved in version 2 0 from the Makefile to MCatNLO inputs The former variable must be set equal to the object file name of the version of HERWIG currently adopted matching the one whose common blocks are included in the files mentioned in sect 2 The variable HWUTI must be set equal to the list of object files that the user needs in the analysis routines 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
33. rst implemented in HERWIG 6 5 the version must be 6 500 or higher On most systems users will need to delete the dummy subroutines UPEVNT UPINIT PDFSET and STRUCTM from the standard HERWIG 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 version of HERWIG currently 6 510 with versions lower than 6 504 problems can be found in attempting to specify the decay modes of single vector bosons through the variable MODBOS Also crashes in the shower phase have been reported when using HERWIG 6 505 and we therefore recommend not to use that version 1 3 Working environment We have written a number of shell scripts and a Makefile all listed under Shell utilities above 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 For Macs running under OSX v10 or higher make can be used instead of gmake 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 1 4 Source and running directories We assume that all the files of the package sit in the same directory which we call the source directory When creating the executable our shell scripts determin
34. t 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 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 boson production a 2 3 effect and the assignment of Agcp 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
35. t file eventually processed by HERWIG Valid entries are 0 and 1 When set to 0 the weights in the event file are 1 When set to 1 they are tw with w a constant such that the sum of the weights gives the total NLO cross section N B These weights are redefined by HERWIG at MC run time according to its own convention see HERWIG manual 12 RNDEVSEED BASES PDFLIBRARY HERPDF HWPATH SCRTCH HWUTI HERWIGVER PDFPATH LHAPATH LHAOFL 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 3 1 Valid entries are PDFLIB LHAPDF and THISLIB In the former two case PDFLIB or LHAPDF is used to compute the parton densities whereas in the latter case the densities are obtained from our self contained faster package If set to DEFAULT HERWIG uses its internal PDF set controlled by NSTRU regardless of the densities adopted at the NLO level If set to EXTPDF HERWIG uses the same PDFs as the NLO code see sect 2 1 The physical address of the directory where the user s preferred version of HERWIG is stored 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
36. ton pair associated Higgs and single top production have been taken from refs 10 11 ref 12 ref 13 ref 14 and ref 15 respectively This documentation refers to MC NLO version 3 3 previous versions 1 0 2 0 2 2 2 3 3 1 and 3 2 are described in refs 16 17 18 19 20 21 respectively Decay angular correlations due to spin correlations in top pair and single top production 22 have been added since version 3 2 Higgs production IPROC 1600 ID now features a more sophis ticated matching procedure 23 which improves the predictions for the pr spectra of the accompanying jets For precise details of version changes see app A 1 A 7 1 1 Mode of operation 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 MCQNLO all of the hard kinematic configurations are generated in advance and stored in a file which we call event file see sect 3 1 the event file is then read by HERWIG which showers and hadronizes each hard configuration Since version 2 0 the events are handled by the Les Houches generic user process interface 24 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 the standard MC The signal to HERWIG that configurations should be read from an event file using the Les Hou
37. y modify this part as customary in MC runs Finally the sample codes mcatnlo_hwanzzz 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 4 2 1 Parton densities Since the knowledge of the parton densities PDF is necessary in order to get the physical cross section a PDF library must be linked The possibility exists to link the now obsolete CERNLIB PDF library PDFLIB or its replacement LHAPDF 28 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 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 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 PDFs These files which can
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