Changes between Version 2 and Version 3 of YETI08


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Timestamp:
04/13/12 09:13:24 (7 years ago)
Author:
md987
Comment:

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  • YETI08

    v2 v3  
    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.
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    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 [wiki: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:Matching this page]). It works as follows.
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    1717Multi-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.
     
    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.
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    23 For 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.
     23For more technical details also have a look at this page about [wiki:Matching Matching]. Also [wiki:MatchChecker this page] has a lot of information about matching/merging and testing your event samples.
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