Status report on the showering of Alpgen events with Herwig++ for ATLAS

Transcription

1 Status report on the showering of Alpgen events with Herwig++ for ATLAS University of Oslo February 203

2 Introduction We want to set up and validate the processing of Alpgen events with Herwig++ in ATLAS, as Herwig++ is expected to gradually take over all the responsibilities of Fortran HERWIG The processing of Alpgen events with Herwig++ has been possible since release For validation of the Alpgen+Herwig++ setup, I use existing parton level Alpgen events from the grid, and compare the showered events to the corresponding (and ) samples and to data using Rivet Some comparison of Herwig++ to Fortran HERWIG is done without underlying event, while comparisons involving data are done with underlying event Herwig++ vs. Fortran HERWIG comparisons without underlying event can be compared to a study performed by the author of the Alpgen interface to Herwig++

3 Systematic differences reported in initial validation by the author W boson p in peak region Mass of first jet dσ/dp W [pb/gev] Fhw hw++ dσ/dm(jet ) [pb/gev] Fhw hw Ratio p W [GeV] Ratio m(jet ) Systematic differences observed in W p T and jet mass distributions These were however shown to be present also when using native matrix elements, and are thus not related to the handling of Alpgen inputs I will not show these distributions, but you can note that these disagreements are present also in my plots

4 W +jets at 8 TeV, no UE Pseudorapidity of leading jet Pseudorapidity of second jet dσ/dη(jet )[pb] 0 2 dσ/dη(jet 2)[pb] dσ/dη(jet 3)[pb] Ratio η(jet ) Pseudorapidity of third jet 0 dσ/dη(jet 4)[pb] Ratio η(jet 2) Pseudorapidityoffourthjet Ratio Ratio η(jet 3) η(jet 4)

5 W +jets at 8 TeV, no UE Ratio dσ/dp (jet 3)[pb/GeV] Ratio dσ/dp (jet )[pb/gev] Transversemomentumofleadingjet p (jet )[GeV] Transversemomentumofthirdjet p (jet 3)[GeV] Ratio dσ/dp (jet 4)[pb/GeV] Ratio dσ/dp (jet 2)[pb/GeV] Transverse momentum of second jet p (jet 2)[GeV] Transverse momentum of fourth jet p (jet 4)[GeV]

6 Choice of strong coupling constant In the default setup of the Alpgen interface to Herwig++, the strong coupling constant for the shower is set to be consistent with the PDF used in the hard scattering (in Alpgen) For our choice of PDF, CTEQ6L, this results in α s (m Z ) = 0.3 and one-loop running However, Fortran HERWIG uses Λ QCD = 80 MeV and two-loop running independent of PDFs Thus, enforcing this coupling choice in Herwig++ may improve agreement between the two

7 W +jets at 8 TeV, no UE, two-loop α s Pseudorapidity of leading jet Pseudorapidity of second jet dσ/dη(jet )[pb] 0 2 dσ/dη(jet 2)[pb] dσ/dη(jet 3)[pb] Ratio η(jet ) Pseudorapidity of third jet 0 dσ/dη(jet 4)[pb] Ratio η(jet 2) Pseudorapidityoffourthjet Ratio Ratio η(jet 3) η(jet 4)

8 W +jets at 8 TeV, no UE, two-loop α s dσ/dp (jet )[pb/gev] Transversemomentumofleadingjet dσ/dp (jet 2)[pb/GeV] Transverse momentum of second jet dσ/dp (jet 3)[pb/GeV] Ratio p (jet )[GeV] Transversemomentumofthirdjet dσ/dp (jet 4)[pb/GeV] Ratio p (jet 2)[GeV] Transverse momentum of fourth jet Ratio Ratio p (jet 3)[GeV] 0 2 p (jet 4)[GeV]

9 Difference from the interface author s study The Herwig++ results from the previous slides have been compared directly to results from the initial validation by the interface author, and good agreement was observed In his study, agreement between Fortran HERWIG and Herwig++ was significantly better (shown in the meeting Dec. 3th) This must be due to some setting in Fortran HERWIG differing between the two studies So far, this is not understood

10 Improved shower model The author of the Alpgen interface to Herwig++ suggested trying out an improvement to the Herwig++ shower based on a paper by Schofield and Seymour The following lines are added to the Herwig++ commands: cd /Herwig/Shower set Evolver:ColourEvolutionMethod set PartnerFinder:PartnerMethod set GtoGGSplitFn:SplittingColourMethod

11 W +jets at 8 TeV, no UE, two-loop α s, improved shower Pseudorapidity of leading jet Pseudorapidity of second jet dσ/dη(jet )[pb] 0 2 dσ/dη(jet 2)[pb] dσ/dη(jet 3)[pb] Ratio η(jet ) Pseudorapidity of third jet 0 dσ/dη(jet 4)[pb] Ratio η(jet 2) Pseudorapidityoffourthjet Ratio Ratio η(jet 3) η(jet 4)

12 W +jets at 8 TeV, no UE, two-loop α s, improved shower Ratio dσ/dp (jet 3)[pb/GeV] Ratio dσ/dp (jet )[pb/gev] Transversemomentumofleadingjet p (jet )[GeV] Transversemomentumofthirdjet p (jet 3)[GeV] Ratio dσ/dp (jet 4)[pb/GeV] Ratio dσ/dp (jet 2)[pb/GeV] Transverse momentum of second jet p (jet 2)[GeV] Transverse momentum of fourth jet p (jet 4)[GeV]

13 Conclusions so far Some difference between Herwig++ and Fortran HERWIG comes from the choice of strong coupling constant There is some difference between the Fortran HERWIG results used by the interface author in his validation and the results from the Athena setup of Fortran HERWIG, which is not fully understood When using an improvement on the shower model and a two-loop α s, Herwig++ produces results that are rather close to Fortran HERWIG in terms of the Rivet analysis MC_WJETS

14 Comparisons to data with underlying event We proceed by showing some comparisons to data using the Rivet analysis ATLAS_202_I08338 First, we compare the three setups, (i.e. Fortran HERWIG + Jimmy for underlying event),, and Alpgen+Herwig++, with the latter set up with the two-loop α s and shower improvement (i.e. the setup that provides results closest to Fortran HERWIG) Following the Z analysis (ATL-COM-PHYS ), we define a k-factor k = σ inclusive i σ gen,npi where the denominator is the sum of the individual parton multiplicity cross sections as given by the Alpgen+X combination, and σ inclusive = 6 nb is the higher order inclusive W e ν cross section. This k-factor is then applied democratically to all parton multiplicity samples.

15 Normalization comparisons We can compare the normalizations obtained from the various Alpgen+X combinations by comparing the resulting k-factors arrives a bit closer to the correct normalization, but all these k-factors seem more or less consistent with a LO generator Generator combination k-factor.96 Alpgen+Herwig

16 Shape comparisons Inclusive Jet Multiplicity Inclusive Jet Multiplicity σ(w + Njetjets)[pb] σ(w + Njetjets)[pb] Njet Njet FirstJetp FirstJetp

17 Shape comparisons 0 FirstJetp (W+ 2jets) 0 FirstJetp (W+ 2jets) FirstJetp (W+ 3jets) FirstJetp (W+ 3jets)

18 Shape comparisons 0 SecondJetp 0 SecondJetp SecondJetp (W+ 3jets) SecondJetp (W+ 3jets)

19 Shape comparisons ThirdJetp ThirdJetp dσ/dht[pb/gev] HT(W+ jets) dσ/dht[pb/gev] HT(W+ jets) HT[GeV] HT[GeV]

20 Shape comparisons HT(W+ 2jets) HT(W+ 2jets) dσ/dht[pb/gev] 0 dσ/dht[pb/gev] HT[GeV] HT[GeV] First Jet Rapidity First Jet Rapidity dσ/dy[pb] dσ/dy[pb] y y

21 Conclusions so far Using the Rivet analysis ATLAS_202_I08338, we find similar results between,, and Alpgen+Herwig++ when the latter is set up with the two-loop α s and shower improvement (i.e. the setup that provides results closest to Fortran HERWIG) Some difference is observed in normalization, with Herwig++ and Fortran HERWIG in good agreement, while Pythia provides somewhat higher total cross section

22 Comparison between different Herwig++ setups Finally, let us compare the different Herwig++ setups regarding strong coupling constant and the shower improvement Reminder: Default α s (as given by PDF): α s (m Z ) = 0.3 and one-loop evolution Two-loop α s (as in Fortran HERWIG): Λ QCD = 80 MeV and two-loop evolution α s Shower improvement k-factor Two-loop yes.95 Two-loop no.22 Default yes.242 Default no.260

23 Shape comparisons Inclusive Jet Multiplicity Inclusive Jet Multiplicity σ(w + Njetjets)[pb] σ(w + Njetjets)[pb] Njet Njet FirstJetp FirstJetp

24 Shape comparisons 0 FirstJetp (W+ 2jets) 0 FirstJetp (W+ 2jets) FirstJetp (W+ 3jets) FirstJetp (W+ 3jets)

25 Shape comparisons 0 SecondJetp 0 SecondJetp SecondJetp (W+ 3jets) SecondJetp (W+ 3jets)

26 Shape comparisons ThirdJetp ThirdJetp dσ/dht[pb/gev] HT(W+ jets) dσ/dht[pb/gev] HT(W+ jets) HT[GeV] HT[GeV]

27 Shape comparisons HT(W+ 2jets) HT(W+ 2jets) dσ/dht[pb/gev] 0 dσ/dht[pb/gev] HT[GeV] HT[GeV] First Jet Rapidity First Jet Rapidity dσ/dy[pb] dσ/dy[pb] y y

28 Conclusions so far The difference between the various Herwig++ setups enter mainly in the normalization (cross sections) When the different setups are all normalized to the same cross section, plots show pretty much compatible results (but not identical, the two-loop α s still seems to be preferred) Using the leading order α s from the PDF is theoretically preferred, but not sure whether such a large k-factor (.25) is acceptable

29 Athena implementations Two Athena implementations currently exist, one running directly on Alpgen.unw files, and another running on.lhe files (produced by a AlpGenToLH conversion, or in the future directly by Alpgen) The interface for.unw files will not be put into Athena due to the fact that it will soon(?) be made obsolete by the upcoming Alpgen release The interface for.lhe files may be put into Athena very soon Default settings for the shower improvement and α s need to be settled Both interfaces can already easily be set up and tested locally on your computer following the instructions here:

30 Conclusions Herwig++ can be used to shower Alpgen events, and gives results very close to Fortran HERWIG depending on the setup Differences between different Herwig++ setups enter mainly through the cross sections, so they seem to all give rather compatible (but not identical) results as long as they are all normalized to the same higher order cross section The best results seem to be obtained using the two-loop α s and shower improvement Two Athena interfaces exist, and the one for.lhe files may soon be put into an official Athena release Before anything can go into central production, input datasets containing.lhe files must be created (for example by converting existing input datasets)

31 BACKUP SLIDES BACKUP SLIDES

32 Additional information Grid datasets for 8 TeV inputs: group.physgener.alpgen x.wenunpx CTEQ6L 8TeV.TXT.mc2* Grid datasets for 7 TeV inputs: group0.phys-gener.alpgen.0768x.wenunpx pt20 7tev.TXT.* Grid datasets for with UE at 7 TeV: mc 7TeV.0768X.AlpgenJimmyWenuNpX pt20.evgen.evnt.e825* Grid datasets for with UE at 7 TeV: mc 7TeV.768X.AlpgenPythiaWenuNpX pt20.evgen.evnt.e028/

33 Additional information Job options for regeneration of 8 TeV Jimmy samples without UE: MC2.0768X.AlpgenJimmy AUET2CTEQ6L WenuNpX.py with additional lines: # Turn UE off in Herwig topalg.herwig.herwigcommand += [ "msflag 0" ] Herwig++ version: 2.6. (through Athena ) Athena version for production without underlying event:

34 Comparisons without k-factors Inclusive Jet Multiplicity Inclusive Jet Multiplicity σ(w + Njetjets)[pb] σ(w + Njetjets)[pb] Njet Njet FirstJetp FirstJetp

35 Comparisons without k-factors 0 FirstJetp (W+ 2jets) FirstJetp (W+ 2jets) FirstJetp (W+ 3jets) FirstJetp (W+ 3jets)

36 Comparisons without k-factors SecondJetp 0 SecondJetp SecondJetp (W+ 3jets) SecondJetp (W+ 3jets)

37 Comparisons without k-factors ThirdJetp ThirdJetp dσ/dht[pb/gev] HT(W+ jets) dσ/dht[pb/gev] HT(W+ jets) HT[GeV] HT[GeV]

38 Comparisons without k-factors HT(W+ 2jets) HT(W+ 2jets) dσ/dht[pb/gev] 0 dσ/dht[pb/gev] HT[GeV] HT[GeV] First Jet Rapidity First Jet Rapidity dσ/dy[pb] dσ/dy[pb] y y