Neutral Diboson Generators

1
Neutral Diboson Generators

• Tom Barber
• Cambridge 15/01/09

2
Introduction

• Overall Aim Repeat CSC study with Sherpa
• Now includes anomalous couplings (AC)
• Want to compare generators for ZZ events
• All generators are Leading Order
• Events generated with CTEQ6L pdf at 14 TeV
• Start with ZZ ? 4l channel...

3
Generators Used

• Pythia
• ffbar ? (g/Z)(g/Z) ? 4l (le,mu,tau)
• Only SM, no anomalous couplings
• Athena interface
• Sherpa
• ffbar ? (g/Z)(g/Z) ? 4l (le,mu,tau)
• Now includes anomalous couplings
• Order electroweak 2
• Jetfinder sqr(20/E_CMS) 1 (cant run without
  this!)
• No Z in event record, need to manually combine
  lepton pairs...
• Athena Interface
• Baur MC
• ffbar ? (g/Z)(g/Z) ? 2e2mu (need to scale up)
• Specifically for AC studies (used in CSC note
  limits)
• No underlying event
• Minimum m(ll) 70 GeV, m(llll) 50 GeV

4
Cross Sections

• Pythia
• s(ZZ?4l (le,m,t)) 159.2 fb
• Out of 100,000 events, 36189 have l e,m and
  m(ll) 70 GeV
• s(ZZ?4l (le,m)) 57.6 fb
• Sherpa
• s(ZZ?4l (le,m,t)) 137.5 fb
• Out of 100,000 events, 47454 have l e,m and
  m(ll) 70 GeV
• s(ZZ?4l (le,m)) 65.2 fb
• Baur
• s(ZZ?2e 2m) 30.5 fb (need to scale by 2)
• s(ZZ?4l (le,m)) 61.1 fb
• MC_at_NLO
• as a reference, no Z mass cut
• s(ZZ?4l (le,m)) 66.77 fb

Origin of Discrepancies?
5
Distributions (Log)
Baur -- Pythia -- Sherpa
6
Distributions (Lin)
Baur -- Pythia -- Sherpa
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Conclusions

• Comments
• Good agreement in tails of pT distributions.
• Sherpa slightly better agreement in pT peak.
• Bad agreement in Z mass tails.
• Follow-up
• Investigate origin of discrepancies tuning?
  hidden cuts? missing something?
• Generate more SM samples with lepton filter
  applied to Pythia and Sherpa.
• Look at ZZ?llnunu channel.
• Investigate anomalous couplings with Sherpa.