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

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

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

Low x meeting, Sinai QCD factorisation inclusive dijet hard scattering QCD matrix element, perturbatively calculated, process dependent Universal diffractive parton densities identical for all processes get PDFs from inclusive diffraction ⇨ predict cross sections for exclusive diffraction universal hard scattering cross section (same as in inclusive DIS) diffractive parton distribution function s → obey DGLAP universal for diffractive ep DIS (inclusive, di-jets, charm) proven for DIS (J.Collins (1998)) not proven for photoproduction!

Low x meeting, Sinai σ diff = flux(x P ) · object (β,Q 2 ) Results from inclusive diffraction β Q2Q2 Reduced cross section from inclusive diffractive data get diffractive PDFs from a NLO (LO) DGLAP QCD Fit to inclusive data from 6.5 GeV 2 to 120 GeV 2 extrapolation of the Fit to lower Q 2 to higher Q 2 gives a reasonably good description of inclusive data from ∼ 3.5 GeV 2 –1600 GeV 2 Regge factorisation is an additional assumption, there is no PROOF!! pomeron flux factor pomeron PDF

Low x meeting, Sinai 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 z P Assuming factorisation holds, the jet and HQ cross sections give better constraint on the gluon density

Low x meeting, Sinai Jet and HQ production Hard scale is E T of the jet or HQ mass Direct access to gluon density Can reconstruct z P in dijet events tests of universality of PDF’s (=QCD factorisation) test of DGLAP evolution

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

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

Low x meeting, Sinai pp  γ*p CDF Tevatron data: 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! Exporting PDFs from HERA to the Tevatron

Low x meeting, Sinai Direct and resolved processes at HERA x γ - fraction of photon’s momentum in hard subprocess DIS (Q 2 >5GeV 2 ) and direct photoproduction (Q 2 ≃ 0): photon directly involved in hard scattering x γ =1 Resolved photoproduction: photon fluctuates into hadronic system, which takes part in hadronic scattering dominant at Q 2 ≃ 0 x γ <1 unsuppressed! suppressed! ? ?

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

Low x meeting, Sinai 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 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 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 Summary  Dijets in DIS & D* cross section: agree with the NLO prediction with the H 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?

18 LPS proton vs Rapidity Gap Large rapidity gap selection: M Y <1.6 GeV and |t|<1 GeV 2 LPS proton selection: M Y = m p extrapolated to |t|<1 GeV 2  Good agreement between two methods and two experiments  Data well described by H1 QCD fit to LRG data ZEUS M x data should be scaled by 0.69 to account for p-diss M.Kapishin, Inclusive diffraction at HERA18

Low x meeting, Sinai H1 and ZEUS