pythia tune when matching with pt-ordered showers, shower kt scheme and W+jets matching parameters

Asked by Rakhi Mahbubani on 2014-02-25

Dear MG/ME team,

In the course of a conversation with Peter Skands, it emerged that the default pythia 6 tune is tailored for virtuality-ordered showers. He strongly recommended I manually change to one of the Perugia tunes (MSTP(5)=350/356) if matching using pt-ordered showers: apparently this is necessary in order to correctly obtain the distribution of the first radiated jets in Drell-Yan at LHC7. I checked the ME2pythia.f file to see if the matching interface automatically enforces this when pt showers are turned on, but I couldn't find anything relevant. Does that mean I need to change the tune myself in the pythia_card? If so, please could you add a statement to that effect in the MG/ME matching notes?

Also, is there a recommended matching scheme/parameter combination for generating W+jets at the LHC?

thanks very much for your help!


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de Visscher (sdevissc) said : #1

Hi Rakhi,

The default tune now in MG+Pythia package is MSTP(81)=20 which are the pt-ordered showers. We never intended to impose specific tunes, but you can always define the one you like. Concerning an LHC tune: the Tune Z2* is used as a standard in CMS (very close to TuneZ2, see for instance I don't know on the side of ATLAS.



Rakhi Mahbubani (r-mahbubani) said : #2

Dear Simon,

I'm sorry, I guess my question was not very clear. It is my impression that the default set of parameters used by pythia for showering (called when merging and matching is done in MadEvent using the MLM procedure with pt-ordered showers) only gives you 'correct' distributions when virtuality-ordered showers are used. In other words the distributions produced by pythia using the default tune with pt-ordered showers turned on don't make much sense. It is necessary in Pythia to change to a set of parameters that gives realistic distributions with pt-ordered showers, i.e to change the tune in the pythia_card when merging and matching with pt-ordered Pythia showers. Which specific tune is used is a matter of choice, as long as it corresponds to pt-ordered showers, which is not the default behaviour in pythia.



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