Changes between Version 1 and Version 2 of Matching


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Timestamp:
04/06/12 16:33:02 (7 years ago)
Author:
trac
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  • Matching

    v1 v2  
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    3 === Matching of jets between MadEvent and [:PythiaInterface:Pythia] ===
     3=== Matching of jets between MadEvent and [wiki:PythiaInterface Pythia] ===
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    5 See also the [:Main/IntroMatching:Introduction to Matching in MadEvent and Pythia].
     5See also the [wiki:Main/IntroMatching Introduction to Matching in MadEvent and Pythia].
    66
    77In MadEvent, three versions of matching are implemented:
     
    99   * MLM matching with cone jets (as in !AlpGen)
    1010   * MLM matching with kt jets (where there are two options for Pythia treatment, the normal MLM procedure or the "Shower kT" scheme)
    11    * CKKW matching with Pythia %$P_T$% shower Sudakov form factors (this option is under development)
     11   * CKKW matching with Pythia $P_T$ shower Sudakov form factors (this option is under development)
    1212
    13 The matching scheme (CKKW or MLM) is chosen by the setting of the parameter ickkw in the run_card.dat (ickkw=0 for no matching, 1 for MLM matching and 2 for CKKW matching). The use of cone jets or kt jets is decided by whether the parameter xqcut (specifying the minimum kt jet measure between jets, i.e. gluons or quarks (except top quarks) which are connected in Feynman diagrams) in the run_card.dat is 0 or not. If xqcut=0, cone jets are used, while if xqcut > 0, kt jet matching is assumed. In this case, ptj and drjj should be set to zero. Note: For most processes, the generation speed can be improcved by setting ptj and mjj to xqcut, which is done automatically if the flag auto_ptj_mjj is set to T. If some jets should not be restricted this way (as in single top or vector boson fusion (VBF) production, where some jets are not radiated from QCD), auto_ptj_mjj should be set to F.
     13The matching scheme (CKKW or MLM) is chosen by the setting of the parameter ickkw in the run_card.dat (ickkw=0 for no matching, 1 for MLM matching and 2 for CKKW matching). The use of cone jets or kt jets is decided by whether the parameter xqcut (specifying the minimum kt jet measure between jets, i.e. gluons or quarks (except top quarks) which are connected in Feynman diagrams) in the run_card.dat is 0 or not. If xqcut=0, cone jets are used, while if xqcut > 0, kt jet matching is assumed. In this case, ptj and drjj should be set to zero. Note: For most processes, the generation speed can be improcved by setting ptj and mjj to xqcut, which is done automatically if the flag auto_ptj_mjj is set to T. If some jets should not be restricted this way (as in single top or vector boson fusion (VBF) production, where some jets are not radiated from QCD), auto_ptj_mjj should be set to F.
    1414
    15 If ickkw>0, MadEvent will cluster each event to find its corresponding "parton shower history". This clustering is done according to the Durham kt algorithm, allowing only clusterings corresponding to Feynman diagrams for the process in question (thereby avoiding e.g. clustering of two gluons to a Z). For each clustered QCD vertex, the scale of alpha_s is set to be the kt jet measure value in that vertex. This corresponds to reweighting each alpha_s to the value it would get in a corresponding parton shower. The clustering value for each final-state parton is printed as a comment for each event in the output LHE event file.
     15If ickkw>0, MadEvent will cluster each event to find its corresponding "parton shower history". This clustering is done according to the Durham kt algorithm, allowing only clusterings corresponding to Feynman diagrams for the process in question (thereby avoiding e.g. clustering of two gluons to a Z). For each clustered QCD vertex, the scale of alpha_s is set to be the kt jet measure value in that vertex. This corresponds to reweighting each alpha_s to the value it would get in a corresponding parton shower. The clustering value for each final-state parton is printed as a comment for each event in the output LHE event file.
    1616
    1717If ickkw=2, MadEvent will also apply a Sudakov suppression factor for each internal parton line, with starting and ending scales corresponding to the scales in the surrounding vertices. Please note that this option is still under development.
     
    2020
    2121   * ickkw: 0 for no matching, 1 for MLM matching and 2 for CKKW matching
    22    * xqcut: minimum jet measure (pT/kT) for QCD partons, if xqcut=0 use cone jet matching, if xqcut> 0 use kt jet matching. This value should be related to the hard scale (e.g. mass of produced particle, HT cut, or similar) in the process, and set to ~(1/6-1/3 x hard scale). Please check that the differential jet rate plots (which are automatically generated if you have MadAnalysis and Root properly installed on your system) are smooth, and check that the cross section does not vary significantly when the xqcut is varied up and down.
     22   * xqcut: minimum jet measure (pT/kT) for QCD partons, if xqcut=0 use cone jet matching, if xqcut> 0 use kt jet matching. This value should be related to the hard scale (e.g. mass of produced particle, HT cut, or similar) in the process, and set to ~(1/6-1/3 x hard scale). Please check that the differential jet rate plots (which are automatically generated if you have MadAnalysis and Root properly installed on your system) are smooth, and check that the cross section does not vary significantly when the xqcut is varied up and down.
    2323   * ptj, ptb, drjj, drbb, drbj: For cone jet matching. Note that for kt jet matching, ptj and ptb should be set to xqcut while drjj, drbj and drbb should be set to 0.
    2424   * fixed_ren_scale, fixed_fac_scale: (default F) %BR% If false, use the highest kt jet measure, or mT of the central produced particles, as factorization and renormalization scales for non-emission vertices (see below)%BR% If true, use the fixed scales as factorization and renormalization scale for non-emission vertices
     
    3131A comment on renormalization and factorization scales: Emission vertices are all QCD vertices where a gluon or light quark (including bottom) are emitted, except the vertex with the highest kt jet measure (e.g. the q-qbar-g vertex in top quark pair production by an s-channel gluon). Only for those vertices is alpha_s evaluated at the jet measure scale. All other vertices are considered to be non-emission vertices. The factorization scale (either the highest kt jet measure or the given fixed scale depending of the value of fixed_fac_scale) is also used as starting scale for the parton shower in the Pythia run. Note that for t-channel singlet exchange processes such as single top or VBF, the factorization scale is set to the pt of the scattered parton on each side of the event. For 4-flavor matching (where b quarks are considered as heavy particles and not as partons), the factorization scale is set to the geometric average of the highest pT_b and the central m_T scale.
    3232
    33 When the event file is read in the [:PythiaInterface:Pythia package], the ickkw parameter is automatically read and matching is turned on, using the routine UPVETO. In this routine, which is called for each event after parton showering but before decays and hadronization, the event is clustered using the corresponding jet clustering scheme (cone jets or kt jets), and the event is rejected or accepted depending on whether the resulting jets correspond to final-state partons in the MadEvent event. For the highest jet multiplicity, extra jets are allowed if they are not harder than the softest MadEvent jet. From MadGraph 5 v. 1.3.18 and Pythia/PGS package v. 2.1.10, non-radiation jets such as the scattered jets in VBF are not included in the matching (but final state radiation from such particles is matched consistently), which allows for variation of the matching scale (xqcut/QCUT) in a consistent way also for such processes.
     33When the event file is read in the [wiki:PythiaInterface Pythia package], the ickkw parameter is automatically read and matching is turned on, using the routine UPVETO. In this routine, which is called for each event after parton showering but before decays and hadronization, the event is clustered using the corresponding jet clustering scheme (cone jets or kt jets), and the event is rejected or accepted depending on whether the resulting jets correspond to final-state partons in the MadEvent event. For the highest jet multiplicity, extra jets are allowed if they are not harder than the softest MadEvent jet. From MadGraph 5 v. 1.3.18 and Pythia/PGS package v. 2.1.10, non-radiation jets such as the scattered jets in VBF are not included in the matching (but final state radiation from such particles is matched consistently), which allows for variation of the matching scale (xqcut/QCUT) in a consistent way also for such processes.
    3434
    35 Either the virtuality-ordered showers (chosen by setting MSTP(81)<20) or the pT-ordered showers (MSTP(81)=20 or 21) can be used in the Pythia run. For the pT-ordered shower, there is an option to use the "shower kT" scheme. This scheme uses information from Pythia about the hardness of the first shower emission to reject events, which means that the same value can be used for QCUT and xqcut.
     35Either the virtuality-ordered showers (chosen by setting MSTP(81)<20) or the pT-ordered showers (MSTP(81)=20 or 21) can be used in the Pythia run. For the pT-ordered shower, there is an option to use the "shower kT" scheme. This scheme uses information from Pythia about the hardness of the first shower emission to reject events, which means that the same value can be used for QCUT and xqcut.
    3636
    3737The Pythia parameters (given in the pythia_card.dat) relevant for matching are:
     
    4646Please see http://arxiv.org/abs/0706.2569, especially sections 2.3 (for MLM matching with cone jet clustering) and 2.4 (for kt jet matching), and http://arxiv.org/abs/0810.5350 (for shower kt), for further details. SHOWERKT (only usable with pt-ordered showers) means that Pythia determines whether to veto events based on the kt values of the hardest shower emission instead of performing jet clustering and comparing with the matrix element. This allows to set QCUT=xqcut, which allows using more of the ME events and therefore improves statistics.
    4747
    48 Note that there are special processes, such as p p &gt; t b~ j + p p &gt; t b~ j j (with 4-flavor matching) which contains a mix of different processes with different highest jet multiplicity - in this case, t-channel single top (with leading order process p p &gt; t b~ j), for which p p &gt; t b~ j j contains only one radiated jet, and s-channel single top (with leading order process p p &gt; t b~) for which p p &gt; t b~ j j contains two radiated jets. In this case, Pythia can not automatically perform the highest multiplicity correctly, and the highest multiplicity (in this case 1 jet) has to be set explicitly in the pythia_card.dat file using MAXJETS=1.
     48Note that there are special processes, such as p p > t b~ j + p p > t b~ j j (with 4-flavor matching) which contains a mix of different processes with different highest jet multiplicity - in this case, t-channel single top (with leading order process p p > t b~ j), for which p p > t b~ j j contains only one radiated jet, and s-channel single top (with leading order process p p > t b~) for which p p > t b~ j j contains two radiated jets. In this case, Pythia can not automatically perform the highest multiplicity correctly, and the highest multiplicity (in this case 1 jet) has to be set explicitly in the pythia_card.dat file using MAXJETS=1.
    4949
    5050Please always check the following after performing matching:
     
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