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Diffractive Dijets in Photoproduction: Cross Section Measurements and Implications from ZEUS Collaboration

This study presents an analysis of diffractive dijets in photoproduction, utilizing data from the ZEUS collaboration (1999-2000). Focusing on single differential cross sections, the research investigates the gluon content of the Pomeron and the resolved photon processes. Key findings highlight the absence of suppression in resolved processes compared to direct processes at leading order (LO). The analysis incorporates Monte Carlo simulations, showing that the data aligns closely with theoretical predictions. The work contributes significant insights into the factorization-breaking effects observed in proton-proton collisions.

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Diffractive Dijets in Photoproduction: Cross Section Measurements and Implications from ZEUS Collaboration

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  1. Diffractive dijets in photoproduction single differential cross sections Shinji Kagawa on behalf of the ZEUS Collaboration Diffractive dijets in photoproduction

  2. Diffractive dijets in photoproduction Parton momenta in the Pomeron ( IP ): reconstructed by jets Dominant process: gluon-intiated process (e.g. boson gluon fusion)  Sensitive to the gluon content of the Pomeron zIPobs longitudinal momentum fraction of the IP taken by the dijet Direct process: total longitudinal momentum of  taken by the dijet Diffractive dijets in photoproduction

  3. The resolved  process in diffractive dijets in photoproduction Resolved  process: • exchanged photon: source of partons  like a hadron • similar to hadron - hadron collision • dominant process at low xobs xobs longitudinal momentum fraction of the  taken by the dijet Diffractive dijets in photoproduction

  4. Factorization breaking in pp collisions  • Possible mechanism of suppression: secondary interactions filling the rapidity gap.  Is there also a suppression of resolved  processes, which are supposedly similar to pp ? • pp collisions at TEVATRON:suppression of diffractive dijetsby a factor of approx. 10(factorization breaking) Diffractive dijets in photoproduction

  5. Suppression of resolved  processes at NLO • M.Klasen and G.Kramer made NLO calculations and compared them to H1 results. • No suppression at LO • At NLO, H1 data is well reproduced when the resolved part is suppressed by a factor of 0.34 • Only suppression at NLO ? Diffractive dijets in photoproduction

  6. Other kinematic variables dijet fraction of electron energy transferred X Direct process longitudinal momentum fraction of the proton transferred invariant mass of the system X dijet X • Reconstruction of jets • longitudinally-invariant kT algorithm Resolved process Diffractive dijets in photoproduction

  7. Kinematic range for measured cross sections • PHP: 0.20 < y < 0.85, Q2 < 1.0 GeV2 • Diffraction: xIP< 0.035 • Dijet: ETjet1(2) > 7.5(6.5) GeV 1.5 < jet1,2 (lab frame)< 2.0 Diffractive dijets in photoproduction

  8. Example event Large rapidity gap (max) characterizing diffractive events e (not detected) dijet Diffractive dijets in photoproduction

  9. Data sample and selection cuts ZEUS 9900 data (77.6 pb-1) Diffractive events selected by requiring (in the lab frame) • E < 1 GeV from pseudorapidity 45 (in the Forward Plug Calorimeter) • no cluster with E  0.4 GeV from pseudorapidity 34 (max cut in the Calorimeter) Large data sample: 10673 events • measurements possible up to high jet ET where NLO calculations are reliable Diffractive dijets in photoproduction

  10. Monte Carlo • RAPGAP version 3.00/00 (Hannes Jung) • Structure functions: GRV-G-LO (  ), H1-fit2 ( IP ) • Only Pomeron contribution generated (no Reggeon) Diffractive dijets in photoproduction

  11. Control plots (I) • MC: reweighted in zIPobs •   (direct) +   (resolved) • is fitted to data in xobs •   = 1.1 • MC has higher max than data •  largest systematic error Diffractive dijets in photoproduction

  12. Control plots (II) • jet1: highest ET jet • jets and MX are well described by MC Diffractive dijets in photoproduction

  13. Differential cross sections in ET jet1and  jet1 • MC area normalized to data by 0.59 • MC describes the shape of jet cross sections well 16% of the cross sections subtracted as p dissociation background (DESY-03-094) Diffractive dijets in photoproduction

  14. Differential cross sections in y and xIP y :electron energy transfer xIP: proton longitudinal momentum transfer  MC describes the shapes of data well Diffractive dijets in photoproduction

  15. Differential cross sections in MX • MX : invariant mass • of system X • Once again, shapes agree Diffractive dijets in photoproduction

  16. Differential cross section in zIPobs • zIPobs: sensitive to the parton distribution of the diffractive exchange (Pomeron) • Data is higher at high zIPobs Diffractive dijets in photoproduction

  17. Differential cross section in xobs resolved enriched direct enriched • Data/MC = 0.59 (flat) • No evidence of a resolved suppression with respect to direct at LO  ? Diffractive dijets in photoproduction

  18. Summary • Single differential cross sections of diffractive dijets in photoproduction measured using ZEUS 9900 data.Adds to the previous ZEUS analysis of 94 data with very high statistics. • Results are fairly consistent with RAPGAP (v3.00/00 H1-fit2) scaled down by 0.59 • Shape of xobs well reproduced  No evidence of a suppression of resolved with respect to direct at LO Diffractive dijets in photoproduction

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