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Pythia versus Alpgen for W+jets (High Mass Dibosons work)

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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Pythia versus Alpgen for W+jets (High Mass Dibosons work)

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  1. Pythia versus Alpgen for W+jets(High Mass Dibosons work) Sarah Allwood 07/12/06

  2. Introduction • Our VBF signal is WWqqWWq’q’: lν qq 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).

  3. Samples • 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.

  4. 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 pythia

  5. Leptonic W, cone R=0.7 • Alpgen through pythia vs Pythia: • Alpgen through Herwig vs Pythia:

  6. Tag jets, cone R=0.7 • Found highest pT jets forward of the W candidates. Alpgen through Pythia Pythia Alpgen through Herwig Pythia

  7. Jet Variables cone R=0.4 alpgen through herwig pythia

  8. W’s and tag jets cone R=0.4 alpgen through herwig vs Pythia

  9. Summary • 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.

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