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Rapidity Gaps in Photoproduction

Rapidity Gaps in Photoproduction. Patrick Ryan University of Wisconsin C. Gwenlan: Oxford University M. Sutton: UCL March 4, 2004. ZEUS Collaboration Meeting DESY. Outline. Introduction Analysis update Comparisons to MC and between analyses Cross Sections and Gap Fractions Summary.

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Rapidity Gaps in Photoproduction

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  1. Rapidity Gaps in Photoproduction Patrick Ryan University of Wisconsin C. Gwenlan: Oxford University M. Sutton: UCL March 4, 2004 ZEUS Collaboration Meeting DESY

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

  3. 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 2p g Remnant f Leading Jet Gap Trailing Jet 0 p Remnant h • 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 < ETCut • Gap indicates color singlet exchange -2.4 2.4

  5. The Gap Fraction Dijet Events with Large Dh and Energy cut All Dijet Events with Large Dh • Singlet • f(Dh) constant in Dh • Non-Singlet • f(Dh) decreases exponentially with Dh • Particle production fluctuations  Gap • Non diffractive exchange Expectation for Behavior of Gap Fraction (J.D. Bjorken, V.Del Durca, W.-K. Tang) fGap fGapn-s fGapSinglet 2 4 3

  6. 96-97 Reprocessed Data HPP Trigger FLT Slot 42 SLT HiEt I/II/III TLT HPP14 (DST bit 77) Offline Cleaning Cuts |zvtx| < 40 cm No Sinistra95 e+ with Pe > 0.9, Ee > 5 GeV, ye < 0.85 0.2 < yjb < 0.85 Jet Selection |h1,2| < 2.4 ½|h1 + h2| < 0.75 2.5 < |h1 - h2| < 4.0 ET1,2 > 6.0, 5.0 GeV (Had) ET1,2 > 5.1, 4.25 GeV (ZUFO) Gap Sample ETGAP < ETCUT ETCUT = 0.5, 1.0, 1.5, 2.0 (Had) ETCUT = 0.6, 1.2, 1.8, 2.4 (ZUFO) Event Selection

  7. 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

  9. Kinematic QuantitiesHERWIG ZUFOS Currently using 56% Direct and 44% Resolved Good agreement between Data and MC

  10. Kinematic QuantitiesPYTHIA ZUFOS Currently using 39% Direct and 61% Resolved Good agreement between Data and MC

  11. 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

  12. 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

  13. 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

  14. 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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