Exclusive processes in diffraction at HERA

1. Alessandro Polini - INFN Bologna on behalf of the H1 and ZEUS Collaborations XXXVI International Symposium on Multiparticle Dynamics Paraty, Rio de Janeiro, Brazil Exclusive processes in diffraction at HERA • Outline: • Introduction • Diffractive Dijet and D* Production • Vector Meson Production and DVCS • Summary and Outlook A. Polini

2. DESY Hamburg (D) Diffraction at HERA HERA • 820-920 GeV p  27 GeV e+/e- • In 2001 machine was upgraded Q2 (photon virtuality): • Q2~0 (<1 GeV2)  Photoproduction (PHP): photon-proton scattering • Q2>1 GeV2  Deep Inelastic Scattering (DIS), probing the structure of the proton Diffraction: • No color flow at the p vertex • Rapidity gap with no activity in the region around the outgoing proton • At HERA ~ 10% of DIS is diffractive A. Polini

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

4. QCD Factorization in Diffraction _ • Non trivial statement in Diffraction (see p-p Tevatron…) • Factorization proven for e±p DIS • HERA: test with dijet and charm (D*) • QCD Hard scales: ET of jet or charm mass • Test of DGLAP evolution A. Polini

5. Diffractive Dijets in DIS Agreement with NLO for H1 and ZEUS: QCD factorization appears to work A. Polini

6. New Combined Fit DPDFs H1: • Measurement of Diffractive Dijet in DIS • New NLO QCD Fit • Extraction of diffractive gluon and quark singlet wide range good accuracy 0.1<zIP<0.9 A. Polini

7. Diffractive Dijets in Photoproduction H1 and ZEUS: • NLO overestimates data by factor 1.6 • Scaling only resolved part doesn’t describe data either • PDF uncertainty? Unlikely, as DIS is described…  what about charm (D*)? A. Polini

8. Diffractive Charm (D*) in DIS H1 99-00 (prel.) H1 2002 Fit (prel.) H1 cmb Fit (prel.) Prel Prel. • NLO QCD: H1 Combined Fit & H1 2002 Fit (μ2= 4mc2+ Q2) • Agreement with results from ZEUS • Data consistent with NLO QCD prediction in shape and normalization • Supporting QCD factorization A. Polini

9. Diffractive Charm in Photoproduction • NLO QCD: • Data consistent with NLO QCD prediction within scale uncertainties • No evidence for suppression of Charm direct PHP • Large NLO uncertainties  Contradiction to dijet results? A. Polini

10. Exclusive Vector Meson Production Regge approach: pQCD approach: A. Polini

11. ρ0 in Photoproduction H1: new measurement HERA II data (2005) Q2 < 4 GeV2 ;20 < W< 90 GeV ~ 250000 ρ0 Candidates - Fits from data from a single experiment - Extract Pomeron trajectory A. Polini

12. ρ0Pomeron Trajectory • Fit single linear trajectory: • Good agreement with • previous ZEUS results • and reduced uncertainty • Significantly smaller slope • than Soft Pomeron • (Donnachie-Landshoff) A. Polini

13. ρ0production at large |t| • H1: HERA-I data • Q2 < 0.001 GeV2 • 75 < W < 95 GeV • 1.5 < |t| < 10 GeV2 • MY< 5 GeV • Good description by • fit with • Two gluon model predictions • don’t describe data • BFKL model gives reasonable • description A. Polini

14. Elastic J/Ψ production J/Ψμ+μ-; J/Ψ e+e- Photoproduction: Q2< 1 GeV2 |t| < 1 GeV2 40 < W < 305 GeV2 Good agreement with measurement by ZEUS Fit W δ δ=0.75 ± 0.03 ± 0.03 MRT sensitive to the shape of the gluon at low x A. Polini

15. Elastic J/Ψ production • Pomeron trajectory shrinkage • Photoproduction: • αP(t) = 1.22 ± 0.010 ± 0.012 + + (0.164 ± 0.028 ± 0.030) GeV-2 · t • Electroproduction: (2<Q2<80 GeV2) • Harder than Soft Pomeron (DL) • Small shrinkage A. Polini

16. J/Ψproduction at large |t| • ZEUS: 50 < W < 150 GeV , MY < 30 GeV, 1< |t| < 20 GeV2, z<0.95 • H1 data below ZEUS at low |t|, agreement at high |t| • DGLAP and BFKL with fixed αS describe |t| dependence • BFKL reproduces rise with W, DGLAP does not A. Polini

17. Deeply Virtual Compton Scattering NLO LO • e+p  e+γ+p • First Diffractive process fully calculable in QCD • No VM wave function uncertainty • Access to Generalized Parton Distributions (GPDs) f(x, ξ, t; μ2) • ξskewedness: p difference between emitted and absorbed parton • QCD: t dependence ebt; ξ and Q2 dynamics by evolution equations A. Polini

18. DVCS Results H1: HERA-II data (2004, 40 pb-1) 30 < Q2 < 80 GeV2, |t| < 1 GeV2 good agreement with HERA-I data Statistical errors reduced Fit Wδ: δ= 1.00 ± 0.16 ± 0.22 A. Polini

19. DVCS Comparison to Predictions Both NLO QCD and Color Dipole Model give good descriptions Measurement sensitive to GPDs Different W dependence? A. Polini

20. Conclusions and Outlook • Diffractive Dijet and charm in DIS support validity of QCD Factorization • H1 combined QCD Fits to inclusive diffractive DIS and Dijets constrain quark and gluon diffractive PDFs in a wide range of fractional momentum • Dijet PHP (direct and resolved) are suppressed by factor 2 w.r.t. NLO QCD  Breaking of QCD factorization in PHP? • Vector Meson and DVCS: • Full set of measurements in PHP and DIS • Transition from soft to hard scattering regime observed • DVCS and heavy VM measurements sensitive to gluon densities • Outlook • Great expectations from HERA II data • HERA II running with polarized e± till mid 2007 A. Polini