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

Sarah Allwood

07/12/06

Introduction
• Our VBF signal is

WWqqWWq’q’:

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).
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.

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

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

Alpgen through Pythia

Pythia

Alpgen through Herwig

Pythia

Jet Variables cone R=0.4

alpgen through herwig

pythia

W’s and tag jets cone R=0.4

alpgen through herwig vs Pythia

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.