29.6.2005 Low x meeting, Sinai 2005 1

Alice Valkárováon behalf of H1 collaborationLOW x meeting 2005, Sinaia

H1 measurements of the structureof diffraction and tests of factorisation

29.6.2005 Low x meeting, Sinai 2005 22

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 → F2

Diffractive DIS: Probe structure of color singlet exchange → F2D

HERA experiments and diffraction

29.6.2005 Low x meeting, Sinai 2005 3

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

22

22

IP WQ

MQ

pq

ppqx X

22

2

XIP MQ

Q

x

x

´

W

ηmax ´

Diffraction kinematics

29.6.2005 Low x meeting, Sinai 2005 4

QCD factorisation

inclusive dijethard scattering QCD matrix element, perturbativelycalculated, process dependent

Universal diffractive parton densitiesidentical for all processes

get PDFs from inclusive diffraction ⇨ predict cross sections for exclusive diffraction

),(),,,()( 2*2

_

* QxtxQxfXpp iIP

D

ipartoni

D i*

Difuniversal hard scattering cross section (same as in inclusive DIS)diffractive parton distribution functions → obey DGLAPuniversal for diffractive ep DIS (inclusive, di-jets, charm)

• proven for DIS (J.Collins (1

998))

• not proven for photoproduction!

29.6.2005 Low x meeting, Sinai 2005 5

σdiff = flux(xP) · object (β,Q2)

Results from inclusive diffractionβ Q2

Reduced cross section frominclusive 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 GeV∼ 2 –1600 GeV2

Regge factorisation is an additional assumption, there is no PROOF!!

),/(),(),,,( 2/

2 QxxftxftxQxf IPIPiIPpIPIP

Di

pomeron flux factor pomeron PDF

29.6.2005 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 givebetter constraint on the gluon density

29.6.2005 Low x meeting, Sinai 2005 7

Jet and HQ production

Hard scale is ET of the jet or HQ massDirect access to gluon densityCan reconstruct zP in dijet events

• tests of universality of PDF’s (=QCD factorisation)

• test of DGLAP evolution

29.6.2005 Low x meeting, Sinai 2005 8

Charm cross section (DIS)

NLO calculations HVQDIS(Harris & Smith)

Good agreement within experimental & theoreticaluncertainties.

Good description of diffractive D* production in DIS (2GeV2 <Q2<100 GeV2)

NLO calculations with PDFs from inclusive diffraction

Factoris

ation holds !

29.6.2005 Low x meeting, Sinai 2005 9

Dijets in DIS

NLO calculations = diffractive extension of DISENT Catani&Seymour (Nucl.Phys.B485 (1997) 29),

interfaced to diffr.PDFs of H1Hadronisation corrections – RAPGAP MC

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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

Good agreement withNLO within exp.&theor.uncertainties

Fact

oris

atio

n hol

ds!

29.6.2005 Low x meeting, Sinai 2005 11

pp

γ*p

CDF Tevatron data:At Tevatron HERA PDF’sdo not work….????

Dijet cross section factor 5-10 lowerthan the QCD calculation using HERAPDFs

?

Break

down o

f fac

toris

atio

n!

Exporting PDFs from HERA tothe Tevatron.........

29.6.2005 Low x meeting, Sinai 2005 12

Direct and resolved processes at HERA

xγ - fraction of photon’s momentum in hard subprocess

DIS (Q2>5GeV2) and direct photoproduction (Q2 0):≃• photon directly involved in hard scattering

• xγ=1

Resolved photoproduction:• photon fluctuates into hadronic system, which takes part in hadronic scattering• dominant at Q2 0≃

• xγ<1

unsuppressed!

suppressed!

hadronsz

jetszOBS

pE

pExx

)(

)(

?

?

29.6.2005 Low x meeting, Sinai 2005 13

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 secondaryinteractions of spectators as the cause:

HERA resolved photoproduction

Secondary interactionsbetween spectators

Jets in photoproductio

n thought to

be

ideal testin

g ground for re

scatterin

g

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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

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Dijets in photoproduction

If only resolved part is suppressed (by factor 0.34, according to Kaidalov et al.)⇨ data are not described by NLO

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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

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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?

18

LPS proton vs Rapidity Gap

Large rapidity gap selection:

MY<1.6 GeV and |t|<1 GeV2

LPS proton selection: MY= mp

extrapolated to |t|<1 GeV2

Good agreement between two methods and two experiments

Data well described by

H1 QCD fit to LRG data

ZEUS Mx data should be scaled by 0.69 to account for p-diss

M.Kapishin, Inclusive diffraction at HERA 18

29.6.2005 Low x meeting, Sinai 2005 19

H1 and ZEUS