Changes between Version 1 and Version 2 of YETI08


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Timestamp:
04/06/12 16:33:02 (8 years ago)
Author:
trac
Comment:

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  • YETI08

    v1 v2  
    1313The biggest difficulty in generating events with multiple jets is to prevent double counting. For example, an event with three jets in the final state could have come from a three parton configuration of which all three showered into different jets, or a 2-parton event of which a third jet is generated by the parton shower (or even an 1 (or zero) parton event with two (or three) jets generated by the shower). The otherway around is also possible. A two jet final state could also have come from a three parton state, of which two parton are belonging to the same jet, and this could therefore lead to double counting with a two-parton sample of which both the partons are showered to a two jet final state.
    1414
    15 There are basically several schemes to avoid this double counting. MadGraph uses a method based on the MLM scheme (also a version of the CKKW method is implemented for more details see [:Software.Matching:this page]). It works as follows.
     15There are basically several schemes to avoid this double counting. MadGraph uses a method based on the MLM scheme (also a version of the CKKW method is implemented for more details see [wiki:Software.Matching this page]). It works as follows.
    1616
    17 Multi-parton events are produced by first generating all the diagrams belonging to the +0, 1, 2, 3, ... jet configurations. Any number of additional particles can be included, the more the 'better' the result will be. In practice, up to 3 jets is the maximum due to the large number of diagrams contributing to these high multiplicity states. For the %$p\bar{p} \to Z/\gamma^* \to \mu^+\mu^- +0,1,2,3\textrm{ jets}$% sample, the proc_card can be found [attachment:proc_card.dat here]. Note that all the diagrams with multiplicities up to three jets will be generated (a couple of thousand diagrams) and this takes in general 1 or 2 hours or so.
     17Multi-parton events are produced by first generating all the diagrams belonging to the +0, 1, 2, 3, ... jet configurations. Any number of additional particles can be included, the more the 'better' the result will be. In practice, up to 3 jets is the maximum due to the large number of diagrams contributing to these high multiplicity states. For the $p\bar{p} \to Z/\gamma^* \to \mu^+\mu^- +0,1,2,3\textrm{ jets}$ sample, the proc_card can be found [attachment:proc_card.dat here]. Note that all the diagrams with multiplicities up to three jets will be generated (a couple of thousand diagrams) and this takes in general 1 or 2 hours or so.
    1818
    19 The second step is, as usual, to generate events by integrating the matrix elements over the phase space. Later-on we will remove events where the 'distance' between two partons is very small, so to improve the effficiency a cut has to be put on the 'kT'-distance between two particles that could form a jet togheter. This 'kT'-distance between particle i and j is defined as %$k_T^{ij}=\sqrt{2\textrm{ min}[p_T^i,p_T^j]^2(\cosh(\eta^i-\eta^j)-\cos(\phi^i-\phi^j))^2}$%. A cut on this variable can be set with the 'xqcut' parameter in the run_card. Altough this is a non-phyical cut, and the final result should be independent of its value, chosing it in an appropriate way improves the efficiency quite a lot. Note that for consistency it has to be smaller than other scales in the process, in particular it should be smaller than the jet measure cut-off (described later). At this stage also the minimum transverse momentum (and possibly invariant mass) of the jets has to be set equal to the xqcut to make sure that there are no gaps in the phase space and increase the efficiency. However, the dR-distance can be set (almost) to zero (e.g. 0.001), because the xqcut already takes care of this.
     19The second step is, as usual, to generate events by integrating the matrix elements over the phase space. Later-on we will remove events where the 'distance' between two partons is very small, so to improve the effficiency a cut has to be put on the 'kT'-distance between two particles that could form a jet togheter. This 'kT'-distance between particle i and j is defined as $k_T^{ij}=\sqrt{2\textrm{ min}[p_T^i,p_T^j]^2(\cosh(\eta^i-\eta^j)-\cos(\phi^i-\phi^j))^2}$. A cut on this variable can be set with the 'xqcut' parameter in the run_card. Altough this is a non-phyical cut, and the final result should be independent of its value, chosing it in an appropriate way improves the efficiency quite a lot. Note that for consistency it has to be smaller than other scales in the process, in particular it should be smaller than the jet measure cut-off (described later). At this stage also the minimum transverse momentum (and possibly invariant mass) of the jets has to be set equal to the xqcut to make sure that there are no gaps in the phase space and increase the efficiency. However, the dR-distance can be set (almost) to zero (e.g. 0.001), because the xqcut already takes care of this.
    2020
    2121The third step is to shower the events. Now we have to make sure that we remove events that 'change the number of jets' to avoid the couple counting. A showered event is clustered to jets using a k_T jet algorithm with a jet measure cut-off (generally speaking 1.5-2 as large as the xqcut). This cut-off makes sure that no new jets are generated from a given parton level multiplicity: if an event generates an extra jet, it is removed from the event sample (except for the highest multiplicity samples, so that also events with 4, 5, 6 or even more jets are generated from the 3 jet sample). Because no new jets are generated for the low multiplicity samples, all double counting is removed. And because the minimum transverse momentum of a jet was equal to the xqcut, the full phase space is covered, and we are not removing too many events.
    2222
    23 For more technical details also have a look at this page about [:Software.Matching:Matching]. Also [:Software.MatchChecker:this page] has a lot of information about matching/merging and testing your event samples.
     23For more technical details also have a look at this page about [wiki:Software.Matching Matching]. Also [wiki:Software.MatchChecker this page] has a lot of information about matching/merging and testing your event samples.
    2424
    2525
    26 ==== %$p\bar{p} \to Z/\gamma^* \to \mu^+\mu^- +$% (no matching) ====
     26==== $p\bar{p} \to Z/\gamma^* \to \mu^+\mu^- +$ (no matching) ====
    2727[attachment:run_01_banner.txt run_01_banner.txt]: Z banner
    28    * at hadron level with jets above %$p_t >10$% GeV [attachment:run_01_pythia_events.lhe.gz event file], [attachment:run_01_pythia_events_plots.ps plots].
    29    * at detector level with jets above %$p_t >10$% GeV [attachment:run_01_pgs_events.lhco.gz event file], [attachment:run_01_pgs_events_plots.ps plots].
     28   * at hadron level with jets above $p_t >10$ GeV [attachment:run_01_pythia_events.lhe.gz event file], [attachment:run_01_pythia_events_plots.ps plots].
     29   * at detector level with jets above $p_t >10$ GeV [attachment:run_01_pgs_events.lhco.gz event file], [attachment:run_01_pgs_events_plots.ps plots].
    3030
    3131
    32 ==== %$p\bar{p} \to Z/\gamma^* \to \mu^+\mu^- +0,1,2,3\textrm{ jets}$% matched ====
     32==== $p\bar{p} \to Z/\gamma^* \to \mu^+\mu^- +0,1,2,3\textrm{ jets}$ matched ====
    3333[attachment:run_01_1_banner.txt run_01_1_banner.txt]: Z+0,1,2,3 jets banner
    34    * at hadron level with jets above %$p_t >10$% GeV [attachment:run_01_1_pythia_events.lhe.gz event file], [attachment:run_01_1_pythia_events_plots.ps plots].
    35    * at detector level with jets above %$p_t >10$% GeV [attachment:run_01_1_pgs_events.lhco.gz event file], [attachment:run_01_1_pgs_events_plots.ps plots].
     34   * at hadron level with jets above $p_t >10$ GeV [attachment:run_01_1_pythia_events.lhe.gz event file], [attachment:run_01_1_pythia_events_plots.ps plots].
     35   * at detector level with jets above $p_t >10$ GeV [attachment:run_01_1_pgs_events.lhco.gz event file], [attachment:run_01_1_pgs_events_plots.ps plots].
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