Interjet Energy Flow Measured in ep Collisions at ZEUS

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Interjet Energy Flow Measured in ep Collisions at
ZEUS
Patrick Ryan University of Wisconsin Aug. 27,
2004

• DPF 2004
• Riverside, CA

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HERA Description
DESY
Photoproduction Event in ep Collisions
Hamburg, Germany
e(k)
H1(ep)
e(k)
ZEUS (ep)
p(p)
Virtuality of Photon

• 820/920 GeV Protons
• 27.5 GeV e- or e
• CMS Energy 300/318 GeV
• Equivalent to 50 TeV fixed target

Inelasticity
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Photoproduction
General Photoproduction
Direct
Resolved

• Photon carries very little 4-momentum (Q2 0)
• Photon is almost real
• Most ep events are photoproduction
• Cross section has 1/Q4 dependence
• Direct g couples directly to a parton in proton
• Resolved
• Fluctuation of g into partonic state
• Parton from g couples to parton in proton

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Diffractive gp in ep Collisions

• Use pQCD to study diffraction in ep collisions
• Hard Diffractive Photoproduction
• Hard High ET Jets (ET 5 GeV)
• Diffractive Gap Between jets, small momentum
  transfer at P vertex
• Photoproduction Q2 0

Standard Diffraction
Hard Diffractive gp
q
Rapidity Gap
t
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Color Non-Singlet and Singlet Exchange in
Resolved gP
Color Non-Singlet Exchange
Color Singlet Exchange
Jet
Jet
Jet
Jet

• Color Non-Singlet Exchange
• Final state partons are color connected
• Space between final state partons filled with
  final state particles
• No Gap between jets
• Color Singlet Exchange
• Final state partons are not color connected
• Space between final state partons empty
• Rapidity Gap between jets

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Topology of Rapidity Gaps
g Remnant
2p
Leading Jet
f
Leading
Trailing Jet
Jet
Gap
g Remnant
Trailing
3
Jet
2p
0
p Remnant
h
f
h
-2.4
2.4
-3
0

• Distance between jet centers Dh
• ETGap Total ET between leading and trailing
  jets
• Gap Event ETGap
• Gap indicates color singlet exchange

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The Gap Fraction
All Dijet Events with Rapidity Gap
Dijet Events with Rapidity Gap and ETGap Expectation for Behavior of Gap Fraction (J.D.
Bjorken, V.Del Durca, W.-K. Tung)
fGap

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

fGapn-s
fGapSinglet
2
4
3
Phys. Rev. D47 (1992) 101
Phys Lett. B312 (1993) 225
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Simulation of gp Events

• PYTHIA 6.1 and HERWIG 6.1
• Shown to match gp
• Use different Fragmentation and Hadronization
  models
• Direct and Resolved MC generated separately
• Resolved MC includes Multi Parton Interactions
• Dir and Res combined by fitting xg distributions
  to Data (coming)
• PDFs
• PDF(p) GRV-LO
• PDF(g) WHIT 2
• Color Singlet Exchange MC
• PYTHIA High-t g
• Purpose is simply to match the data
• Note Rapidity Gap not due to photon exchange
• HERWIG BFKL
• Uses BFKL Pomeron as exchange object in Rapidity
  Gap events

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Event Selection and xg Fitting

• ZEUS 96-97 Data
• Luminosity 38 pb-1
• Offline Cleaning Cuts
• zvtx
• No e with Ee 5 GeV, ye
• 0.2
• Jet Selection
• ET1,2 5.1, 4.25 GeV
• h1,2
• ½h1 h2
• (Spx)2 (Spy)2 / SET
• 2.5
• 4 Gap Samples
• ETGAP
• 70,000 Inclusive Events

• xg Fraction of g momentum involved in collision

xg Fit to Data
Direct Resolved
Direct
46 Direct 54 Resolved
Mixing used in all calculations
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Valid Simulation of gp by HERWIG
h of Leading Jet
Highest ET Jet

• Data well described by HERWIG

h of Trailing Jet
Cut 0.2
Direct Resolved
Direct
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Energy in the Gap
PYTHIA
HERWIG
2.6 Color Singlet
4.8 Color Singlet

• Addition of CS MC gives better agreement at low
  ETGap
• Enough CS added to match Data in lowest bin
• HERWIG agrees better than PYTHIA with Data (used
  in next plots)
• Agreement can be improved by tuning input
  parameters

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Inclusive and Gap Cross Sections
Compare gp Data to HERWIG
Error bars show statistical errors only
Inclusive
Addition of 4.8 color singlet MC improves
agreement with data
ETGap Gap Fraction
Ratio of above plots
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Gap Fractions for Different Gap ET
ETGap ETGap

Observed excess of Data over MC without CS
exchange

Data has better agreement with MC (95.2) CS
(4.8)

Evidence that CS exchange is occurring

ETGap
ETGap
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Tevatron and H1 Results
CDF D0 CSE Fractions

• Fraction of CSE at Tevatron
• vs 1800 GeV
• CDF 1.13 0.12(stat) 0.11(sys)
• D0 Rising Slightly
• Consistent within errors
• vs 630 GeV
• CDF 2.4 0.7 0.6
• ZEUS 4.8 at vs 300 GeV
• Gap Fraction at H1
• Consistent with ZEUS within errors
• 6.6 pb-1 of Lumi

• H1

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Summary

• Conclusions on gp with Rapidity Gap
• HERWIG BFKL and PYTHIA High-t g describe data
• Evidence for Color Singlet Exchange
• 3-5 of CSE added to data improves match at high
  Dh
• ZEUS results consistent with H1 within errors
• ZEUS observes larger CSE than CDF/DO at lower vs
• CDF/D0 1/2.4 CSE at 1800/630 GeV
• Next steps
• Include 98-2000 Data
• 3x higher statistics
• Can go to higher jet ET ? Less sensitivity to
  underlying event models
• Study properties of color singlet exchange