Rapidity Gaps in Photoproduction

1
Rapidity Gaps in Photoproduction

• ZEUS Collaboration Meeting
• DESY

Patrick Ryan University of Wisconsin C.
Gwenlan Oxford University M. Sutton
UCL March 4, 2004
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Outline

• Introduction
• Analysis update
• Comparisons to MC and between analyses
• Cross Sections and Gap Fractions
• Summary

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Motivation

• Use pQCD to study a diffractive (soft) process
• Hard Diffractive Photoproduction
• Hard High ET Jets
• Diffractive Gap Between jets
• Photoproduction Q2 0

Color Singlet Particle
4
Topology of Rapidity Gaps
g Remnant
2p
f
Leading
Jet
Gap
Trailing
Jet
0
p Remnant
h
-2.4
2.4

• Jets found using kT inclusive algorithm
• Distance between jet centers Dh
• ETGap Sum of ET of jets between leading and
  trailing jets
• Gap Event ETGap lt ETCut
• Gap indicates color singlet exchange

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The Gap Fraction
Dijet Events with Large Dh and Energy cut
All Dijet Events with Large Dh
Expectation for Behavior of Gap Fraction (J.D.
Bjorken, V.Del Durca, W.-K. Tang)
fGap

• Singlet
• f(Dh) constant in Dh
• Non-Singlet
• f(Dh) decreases exponentially with Dh
• Particle production fluctuations ? Gap
• Non diffractive exchange

fGapn-s
fGapSinglet
2
4
3
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Event Selection

• 96-97 Reprocessed Data
• HPP Trigger
• FLT Slot 42
• SLT HiEt I/II/III
• TLT HPP14 (DST bit 77)
• Offline Cleaning Cuts
• zvtx lt 40 cm
• No Sinistra95 e with
• Pe gt 0.9, Ee gt 5 GeV, ye lt 0.85
• 0.2 lt yjb lt 0.85

• Jet Selection
• h1,2 lt 2.4
• ½h1 h2 lt 0.75
• 2.5 lt h1 - h2 lt 4.0
• ET1,2 gt 6.0, 5.0 GeV (Had)
• ET1,2 gt 5.1, 4.25 GeV (ZUFO)
• Gap Sample
• ETGAP lt ETCUT
• ETCUT 0.5, 1.0, 1.5, 2.0 (Had)
• ETCUT 0.6, 1.2, 1.8, 2.4 (ZUFO)

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Monte Carlo

• HERWIG 6.1
• ZEUS-AMADEUS version
• Separately generated LO-Dir and LO-Res (with MPI)
• PDFs
• Proton GRV-LO
• Photon WHIT 2
• PYTHIA 6.1
• Stand-alone version
• Separately generated LO-Dir and LO-Res (with MPI)
• PDFs
• Proton GRV-LO
• Photon WHIT 2
• PYTHIA High-t g exchange
• No MPIs
• PDFs
• Proton GRV-LO
• Photon WHIT 2
• Note HERWIG 5.9 Stand-alone and PYTHIA 6.1
  disagree

8
Analysis Update

• Improvements since last meeting
• Color singlet exchange MC added to PYTHIA
• Large increase in HERWIG statistics
• Reweighting to Xgobs
• Calculations of purities and efficiencies

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Kinematic QuantitiesHERWIG ZUFOS
Currently using 56 Direct and 44 Resolved
Good agreement between Data and MC
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Kinematic QuantitiesPYTHIA ZUFOS
Currently using 39 Direct and 61 Resolved
Good agreement between Data and MC
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Purities, Efficiencies, and Acceptances

• Purity (detector.AND.generator)/detector
• Efficiency (detector.AND.generator)/generator
• Acceptance detector/generator

PYTHIA higher than HERWIG for Purities and
Acceptances
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Comparison Between AnalysesData
Inclusive Sample
Gap Sample
All Dijet Events with Large Dh
Dijet Events with Large Dh and E Cut
P. Ryan - C. Gwenlan
Statistical Errors Only
Very good agreement between the analyses
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Comparison between AnalysesCorrected Cross
Section
ETCUT 1 GeV
P. R. Reprocessed MC
C. G Unreprocessed MC
Excess of data in highest Dh bin possible
evidence of color singlet exchange
PYTHIA 98(Dir Res) 2 Color Singlet
Need more Color Singlet contribution to agree
with data at high Dh
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Summary

• Conclusions
• Good agreement between analyses of P. Ryan and C.
  Gwenlan for cross sections and gap fractions
• 15 difference between HERWIG and PYTHIA Cross
  Sections
• Excess observed over standard QCD prediction
• Evidence of color singlet exchange?
• Plans
• Vary contribution MC color singlet exchange
• Further improve agreement between analyses
• Include 96 HERWIG
• Include larger data sample (98-2000 Data)
• Can go to higher jet ET
• Less sensitivity to underlying event models
• Systematic Errors and Energy Corrections
• Paper to be written soon

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Extra 1
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