pythia versus alpgen for w jets high mass dibosons work
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Pythia versus Alpgen for W+jets (High Mass Dibosons work). Sarah Allwood 07/12/06. Introduction. Our VBF signal is WWqq WWq’q’:. l ν. qq. For high m WW , this is one jet with some substructure for cone Δ R =0.7 or k T R=0.5, and not quite 2 jets for cone Δ R =0.4.

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  • Our VBF signal is



For high mWW, this is one jet with some substructure for cone ΔR =0.7 or kT R=0.5, and not quite 2 jets for cone

ΔR =0.4.

  • In this high mass case, pTW ≥ 300GeV whilst pTtag jet>20GeV.
  • W + 3jets is an important background.
  • Alpgen can make W + up to 6 jets.
  • Pythia produces W + 1jet as the hard process; extra jets can come from parton showers ( tag jets could be faked by ISR).
  • Generation cuts for Alpgen:
    • W decays leptonically: pTlep >100GeV, |ηlep|<2.5 ETmiss>100GeV, dR(l-j)>0.
    • pTjet>20GeV, |ηjet|<5, dR(j-j)> 0.7
    • njets = 3
    • …and then through Herwig / Pythia 6.323
  • Generation cuts for Pythia 6.323:
    • W decays leptonically
    • min pT of hard scatter (ckin 3) 250GeV
  • …followed by ATLFAST (11.0.42) with massive cone algorithm.

5618 unweighted events for Alpgen: σ = 3.46±0.17 pb through Herwig

σ = 3.29 pb through Pythia.

100000events for Pythia: σ = 13.53 pb.

jet variables cone r 0 7
Jet variables cone R=0.7

Required NJET>2, ptjet>20GeV, pt lepton>100GeV, ptmiss>100GeV, pt leptonic W>300GeV, pt hadronic W >300GeV.

alpgen through pythia


leptonic w cone r 0 7
Leptonic W, cone R=0.7
  • Alpgen through pythia vs Pythia:
  • Alpgen through Herwig vs Pythia:
tag jets cone r 0 7
Tag jets, cone R=0.7
  • Found highest pT jets forward of the W candidates.

Alpgen through Pythia


Alpgen through Herwig


jet variables cone r 0 4
Jet Variables cone R=0.4

alpgen through herwig


w s and tag jets cone r 0 4
W’s and tag jets cone R=0.4

alpgen through herwig vs Pythia

  • Started a study of Alpgen versus Pythia for W+3jets as a background to high mWW VBF.
  • Possible similarities:
    • After lepton pT>100GeV, W pT>300GeV and tag jet cuts Pythia W+jets and Alpgen W+3jets show similar numbers and tag jet distributions.
  • Possible differences:
    • Alpgen shows a slightly harder pT distribution on additional jets.
    • dR(leptonicW, hadronicW)
  • The cuts used at generation are quite different between Alpgen and Pythia – it’s possible I haven’t completely removed those differences in my ATLFAST cuts.
  • Next:
    • Pythia 6.4 rather than 6.323.
    • more alpgen events, looser cuts, contributions from W+2jets, W+4jets?
    • generator level study
    • different jet algorithm
    • hadronic W cuts, central jet veto.