Alice Valk árová on behalf of H1 collaboration LOW x meeting 2005, Sinaia

1. Alice Valkárová on behalf of H1 collaboration LOW x meeting 2005, Sinaia H1 measurements of the structureof diffraction and tests of factorisation Low x meeting, Sinai 2005

2. HERA experiments and diffraction ´ HERA:~10% of low-x DIS events are diffractive  study QCD structure of high energy diffraction with virtual photon DIS: Probe structure of proton → F2 e´ e ´ Diffractive DIS:Probe structure of color singlet exchange → F2D Low x meeting, Sinai 2005 2

3. Diffraction kinematics Large rapidity gap between leading proton p´ and X ηmax ´ W ´ Momentum fraction of proton carried by color singlet exchange: Momentum fraction of color singlet carried by struck quark: Low x meeting, Sinai 2005

4. QCD factorisation get PDFs from inclusive diffraction ⇨predict cross sections for exclusive diffraction inclusive dijet 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 • universal hard scattering cross section (same as in inclusive DIS) • diffractive parton distribution functions→obey DGLAP • universal for diffractive ep DIS (inclusive, di-jets, charm) Low x meeting, Sinai 2005

5. Results from inclusive diffraction Q2 β Regge factorisation is an additional assumption, there is no PROOF!! pomeron flux factor pomeron PDF σdiff = flux(xP) · object (β,Q2) Reduced cross section from inclusive diffractive data • get diffractive PDFs from a NLO (LO) • DGLAP QCD Fit to inclusive data from • 6.5 GeV2 to 120 GeV2 • extrapolation of the Fit • to lower Q2 • to higher Q2 • gives a reasonably good description of • inclusive data from ∼3.5 GeV2 –1600 GeV2 Low x meeting, Sinai 2005

6. Diffractive Parton Densities • determined from NLO QCD • analysis of diffractive structure • function • more sensitive to quarks • gluons from scaling violation, • poorer constraint • gluon carries about 75% of pome- • ron momentum • large uncertainty at large zP Assuming factorisation holds, the jet and HQ cross sections give better constraint on the gluon density Low x meeting, Sinai 2005

7. Jet and HQ production Hard scale is ET of the jet or HQ mass Direct access to gluon density Can reconstruct zPin dijet events • tests of universality of PDF’s • (=QCD factorisation) • test of DGLAP evolution Low x meeting, Sinai 2005

8. Charm cross section (DIS) Good agreementwithin experimental & theoretical uncertainties. Factorisation holds ! NLO calculations with PDFs from inclusive diffraction NLO calculations HVQDIS (Harris&Smith) Good description of diffractive D* production in DIS (2GeV2 <Q2<100 GeV2) Low x meeting, Sinai 2005

9. Dijetsin DIS NLO calculations = diffractive extension of DISENT Catani&Seymour (Nucl.Phys.B485 (1997) 29), interfaced to diffr.PDFs of H1 Hadronisation corrections – RAPGAP MC Low x meeting, Sinai 2005

10. Dijets in DIS • NLO corrections to LO • are significant – factor 1.9 • excess at high xγ is • kinematically connected • with the lack of events with • ηlab of jets < -0.4 in • comparison with NLO Factorisation holds! Good agreement with NLO within exp.&theor. uncertainties Low x meeting, Sinai 2005

11. CDF Tevatron data:  γ*p pp Exporting PDFs from HERA to the Tevatron......... At Tevatron HERA PDF’s do not work….???? Dijet cross section factor 5-10 lower than the QCD calculation using HERA PDFs Breakdown of factorisation! ? Low x meeting, Sinai 2005

12. Direct and resolved processes at HERA xγ - fraction of photon’s momentum inhard subprocess • DIS (Q2>5GeV2) and direct photoproduction (Q2≃0): • photon directly involved in hard scattering • xγ=1 ? unsuppressed! • Resolved photoproduction: • photon fluctuates into hadronic system, which • takespart in hadronic scattering • dominant at Q2≃0 • xγ<1 ? suppressed! Low x meeting, Sinai 2005

13. Photoproduction ashadronic process HERA resolved photoproduction Secondary interactions between spectators Jets in photoproduction thought to be ideal testing ground for rescattering Typical models that describe suppression at Tevatron assume secondary interactions of spectators as the cause: resolved contribution expected to be suppressed by factor 0.34 (Kaidalov,Khoze,Martin,Ryskin:Phys.Lett.B567 (2003),61) Low x meeting, Sinai 2005

14. Dijets in photoproduction The same kinematical region as for DIS • NLO overestimates the cross section by • factor ∼2 • both direct and resolved are suppressed • RAPGAP LO – good description Low x meeting, Sinai 2005

15. Dijets in photoproduction If only resolved part is suppressed (by factor 0.34, according to Kaidalov et al.) ⇨ data are not described by NLO Low x meeting, Sinai 2005

16. Ratio:data over NLO prediction • no suppression observed for DIS • overall suppression factor of • about 2 observed for both • resolved and direct • components in photoproduction • suppression is independent of the • cms energy W Low x meeting, Sinai 2005

17. Summary • Dijets in DIS & D* cross section: • agree with the NLO prediction with the H1 2002 diffractive pDFs • factorisation holds (assuming PDF is correct) • Dijets in photoproduction: • to investigate the puzzle of disagreement of HERA/Tevatron data • (expectation: resolved will be suppressed and direct not) • data are half of NLO prediction – both resolved and direct are • suppressed ⇨ conflict with the theoretical expectation • More ideas? Low x meeting, Sinai 2005