Exclusive processes in diffraction at HERA

$: Exclusive processes in diffraction at HERA$
ISMD 2006 Paraty, Rio de Janeiro, Brazil 1 A. Polini

Exclusive processes in diffraction at HERAExclusive processes in diffraction at HERA

XXXVI International Symposium on Multiparticle Dynamics

Paraty, Rio de Janeiro, Brazil

Alessandro Polini - INFN Bolognaon behalf of the

H1 and ZEUS Collaborations

Outline: Introduction Diffractive Dijet and D* Production Vector Meson Production and DVCS Summary and Outlook


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

Diffraction at HERADiffraction at HERA

DESY Hamburg (D)

IPz

zL x1

p

p'x


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 η max

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

Diffractive Event SelectionDiffractive Event Selection

)ln(tan- η 2θ e p


QCD Factorization in DiffractionQCD 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

_


Diffractive Dijets in DISDiffractive Dijets in DIS

Agreement with NLO for H1 and ZEUS:

QCD factorization appears to work


New Combined Fit DPDFsNew 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


Diffractive Dijets in PhotoproductionDiffractive 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*)?


H1 99-00 (prel.)

H1 2002 Fit (prel.)

H1 cmb Fit (prel.)

Diffractive Charm (D*) in DISDiffractive Charm (D*) in DIS

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

Prel.

Prel


Diffractive Charm in PhotoproductionDiffractive 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?


Exclusive Vector Meson ProductionExclusive Vector Meson Production

pQCD approach:

Regge approach:


ρρ00 in Photoproduction in PhotoproductionH1: 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


ρρ00 Pomeron TrajectoryPomeron Trajectory

Fit single linear trajectory:

Good agreement with previous ZEUS results and reduced uncertainty

Significantly smaller slope than Soft Pomeron (Donnachie-Landshoff)


ρρ00 production at large |production at large |t|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


Elastic Elastic J/J/ΨΨ production 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


Elastic Elastic J/J/ΨΨ production 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


J/J/ΨΨ production at large production at large |t||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


Deeply Virtual Compton ScatteringDeeply Virtual Compton Scattering

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) ξ skewedness: p difference between emitted and absorbed parton QCD: t dependence ebt; ξ and Q2 dynamics by evolution equations

NLOLO


DVCS ResultsDVCS 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


DVCS Comparison to PredictionsDVCS Comparison to PredictionsBoth NLO QCD and Color Dipole Model give good descriptions

Measurement sensitive to GPDs

Different W dependence?


Conclusions and OutlookConclusions 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

Exclusive processes in diffraction at HERA

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