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Diffractive dijets at HERA

Diffractive dijets at HERA. Representing H1 and ZEUS experiments. Alice Valk árová Charles University, Pra gue. HERA collider experiments. 27.5 GeV electrons/positrons on 920 GeV protons →√s=318 GeV two experiments: H1 and ZEUS HERA I: 16 pb -1 e-p, 120 pb –1 e+p

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Diffractive dijets at HERA

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  1. Diffractive dijets at HERA Representing H1 and ZEUS experiments Alice Valkárová Charles University, Prague

  2. HERA collider experiments • 27.5 GeV electrons/positrons on 920 GeV protons →√s=318 GeV • two experiments: H1 and ZEUS • HERA I: 16 pb-1 e-p, 120 pb–1 e+p • HERA II: ∼ 550 pb-1, ∼ 40% polarisation of e+,e- DIS:Probe structure of proton → F2 e Diffractive DIS: Probe structure of color singlet exchange → F2D

  3. W Diffraction and diffraction kinematics HERA:~10% of low-x DIS events are diffractive ´ momentum fraction of color single exchange My fraction of exchange momentum, coupling to γ* t 4-momentum transfer squared

  4. 1) Proton Spectrometers: ZEUS: LPS (1993-2000) H1: FPS (1995-2008),VFPS (2004-08) t measurement access to high xIP range free of p-dissociation background at low xIP small acceptance  low statistics ☠ 2) Large Rapidity Gap, H1, ZEUS: Require no activity beyond ηmax t not measured, some p-diss background ☠ 3) Mx method, ZEUS: Diffractive vs non-diffractive: exponential fall off vs constant distribution in ln Mx2 Some p-diss contribution ☠ diff. non-diff Diffractive Event Selection p e

  5. QCD factorization Get PDF from inclusive diffraction predict cross sections for exclusive diffraction Hard scattering QCD matrix element,perturbatively calculated, process dependent • proven for DIS (J.Collins (1998)) • not proven for photoproduction! Universal diffractive parton densities, identical for all processes → DPDFs – obey DGLAP, universal for diff. ep DIS (inclusive,dijet,charm) → universal hard scattering cross section (same as in inclusive DIS)

  6. Proton vertex (Regge) factorization Additional assumption – there is no proof! pomeron flux factor pomeron PDF Exctracted from inclusive diffraction! Fits 2006 A and B

  7. H1 2006 fits -results z (zIP)- longitudinal momentum fraction of gluon rel. to colorless exchange Two fits, A and B Fit B: gluon parameterised as a constant at starting scale • quarks are very stable • gluons carries ~75% of • pomeron momentum • gluons similar at low z • no sensitivity to gluon • at high z

  8. Dijets in diffractive DIS, H1 • < Q2 < 80 GeV2 • 0.1 < y < 0.7 • xIIP < 0.03 • P*tjet1 > 5.5 GeV • P*tjet2 > 4 GeV • -3. < η*jets < 0. Data 99/00 zIP is the most sensitive variable to test gluonic part of DPDFs – difference between fits A and B at high zIP. Data agree with NLO prediction, NLO with fit B is more close to data

  9. Dijets in diffractive DIS, H1 For zIP < 0.4 NLO predictions using fits 2006 A and B agree with data very well Combined QCD fit for inclusive and dijet DIS data…..

  10. Largest difference Largest difference H1 QCD jet fit Low sensitivity of fits to inclusive cross section to gluon PDF especially at large zIP→ use jets to combined fits! JHEP 0710:042,2007

  11. Dijets in diffractive DIS, ZEUS Eur.Phys.J.C52: 83 (2007) E*tjet1 > 5 GeV Data 99/00 R=data/NLO(ZEUS LPS) The best agreement of data and NLO for H1 2006 fit B and MRW 2006 H1, ZEUS: for DIS dijets factorization holds…..

  12. γ*p  pp Hadron-hadron collisions Exporting DPDFs from HERA to Tevatron….. xIP integrated effective DPDFs from CDF single diff. dijets (run I) If for HERA incl. data rapidity gap survival factor =1, at Tevatron ~0.1 Factorization broken by β-dependent factor ~ 10 !

  13. resolved photoproduction (Q2≃0): photon fluctuates into hadronic system,which takespart in hadronicscattering,dominant at Q2≃0 xγ<1 Expect: gap survival probability < 1 Theory (Kaidalov at al) ~0.34 Photoproduction, γ⋆p, Q2→0 xγ - fraction of photon’s momentum inhard subprocess direct photoproduction (Q2≃0): photon directly involved in hard scattering xγ=1 Expect: gap survival probablity= 1

  14. One year ago…. H1: Etjet1> 5 GeV suppression of factor ~0.5 ZEUS: Etjet1 > 7.5 GeV weak (if any) suppression (0.6-0.9) Neither collaboration sees difference between the resolved and direct regions, in contrast to theory! Possible explanation of differences between H1 and ZEUS (DIS 2007) Different phase space of both analyses ……..?

  15. Et dependence of suppression? H1 ZEUS From the DIS 2008 talk of W.Slomiński, ZEUS results H1 and ZEUSobserve the data have harder Et slope than NLO

  16. Double ratios of γ⋆p & DIS(data/NLO) H1 collab. Eur.Phys.J,C51 (2007),549 Very useful – full or partial cancellation of many uncertainties (energy scales for data, DPDFs used…etc ). There is no clear W dependence but what about Etjet1 dependence?? Etjet1>5 GeV

  17. Double ratio of Data/NLO for photoproduction and DIS → uncertainties of diffractive PDFs cancel! Figure derived from published results H1 - ratio photoproduction/DIS Double ratio is within errors Et dependent!

  18. Tagged photoproduction, luminosity 3x larger than for 97 diffractive events found by Large Rapidity Gap method (LRG) New H1 analysis – data 99/00

  19. Two cut scenarios To crosscheck previous H1 results To approach closest to ZEUS cuts

  20. 2 programs for NLO calculations, 3 sets of DPDFs: Frixione/Ridolfi → H1 2006 Fit A H1 2006 Fit B H1 2007 Fit Jets → Kramer/Klasen → H1 2006 Fit B Theoretical predictions inclusive measurement DIS dijets The aim was to estimate the effect on DPDF used and to crosscheck NLO programs

  21. Lower Et cut scenario Integrated survival probabilities (ISP) Within errors no difference in ISP using different DPDFs No difference in survival probabilities for resolved and direct regions of xγ, like in previous H1 and ZEUS analyses

  22. ZEUS – W.Slominski Lower Et cut scenario Another hint of Et harder slope for data than NLO Hadronization corrections δhadr=MC(hadr)/MC(parton)

  23. Higher Et cut scenario Now much more „direct-like“ events than in low Et analysis, peak at higher xγ Integrated survival probabilities (ISP) Larger ISP than for lower Et cut scenario → more close to ZEUS results!!!

  24. ZEUS – W.Slomiński Higher Et cut scenario Et dependencenot excluded but cannot be independently verified

  25. What DPDF is the „best“? The most sensitive variable → zIP Fits 2006 A,B valid only to zIP =0.8 Fit 2007 jets to zIP = 0.9 Fits 2006 A and 2007Jets represent extremes. Fit 2006 B is in the middle….

  26. Summary • new H1 dijet photoproduction data – older H1 results confirmed • – within errors isgap survival probability (GSP) ~ 0.5. • in higher Et cut scenario (similar to ZEUS)GSP is ~ 0.6, • more close to ZEUS results. • hint that GSP is dependent on Et of the leading jet, • for low Et jets seems to be suppression more significant. • the evidence that GSP is not different for direct and • resolved events remains (originally not expected ) • to understand better diffractive dijets in photoproduction is • challenging!!!

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