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Diffractive dijets at HERA

Alice Valkárová Charles University, Prague

Representing H1 and ZEUS experiments

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HERA collider experiments• 27.5 GeV electrons/positrons on 920 GeV protons →√s=318 GeV• two experiments: H1 and ZEUS• HERA I: 16 pb-1 e-p, 120 pb–1 e+p• HERA II: ∼ 550 pb-1, ∼ 40% polarisation of e+,e-

e

DIS: Probe structure of proton → F2

Diffractive DIS: Probe structure of

color singlet exchange → F2D

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22

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IP WQ

MQ

pq

ppqx X

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2

XIP MQ

Q

x

x

W

´HERA: ~10% of low-x DIS events are diffractive

Diffraction and diffraction kinematics

momentum fraction of color single exchange

fraction of exchange momentum, coupling to γ*

2, )( ppt t

4-momentum transfersquared

My

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1) Proton Spectrometers: ZEUS: LPS (1993-2000) H1: FPS (1995-2008),VFPS (2004-08)t measurementaccess to high xIP rangefree of p-dissociation background at low xIP

small acceptance low statistics ☠

2) Large Rapidity Gap, H1, ZEUS:Require no activity beyond η max

t not measured, some p-diss background ☠

3) Mx method, ZEUS:Diffractive vs non-diffractive: exponential fall off vs constant distribution in ln Mx

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Some p-diss contribution ☠

2ln2ln

xMB

x

CeDMd

dN

diff. non-diff

Diffractive Event Selection

)ln(tan- η 2θ

e p

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QCD factorizationGet PDF from inclusive diffraction predict cross sections for exclusive diffraction

Hard scattering QCDmatrix element,perturbativelycalculated, process dependent

Universal diffractive partondensities, identical for all processes

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

_

* QxtxQxfXpp iIP

D

ipartoni

D Dif

i*

→ DPDFs – obey DGLAP, universal for diff. ep DIS (inclusive,dijet,charm)

→ universal hard scattering cross section (same as in inclusive DIS)

• proven for DIS (J.

Collins (1

998))

• not proven for p

hotoproduction!

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Proton vertex (Regge) factorization

1)(2/ ),( tIP

Bt

IPpIP xe

txf

Additional assumption – there is no proof!

pomeron flux factor pomeron PDF

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

2 QxxftxftxQxf IPIPiIPpIPIP

Di

Exctracted from inclusive diffraction!Fits 2006 A and B

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H1 2006 fits -results

Two fits, A and BFit B: gluon parameterised as aconstant at starting scale

• quarks are very stable• gluons carries ~75% of pomeron momentum• gluons similar at low z• no sensitivity to gluon at high z

z (zIP)- longitudinal momentum fraction of gluon rel. tocolorless exchange

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Dijets in diffractive DIS, H1

zIP is the most sensitive variable to test gluonic part of DPDFs –difference between fits A and B at high zIP.

Data agree with NLO prediction, NLO with fit B is more close to data

4 < Q2 < 80 GeV2

0.1 < y < 0.7xIIP < 0.03

P*tjet1 > 5.5 GeVP*tjet2 > 4 GeV-3. < η*jets < 0.

Data 99/00

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Dijets in diffractive DIS, H1

For zIP < 0.4 NLO predictionsusing fits 2006 A and B agree with data very well

Combined QCD fit for inclusive and dijet DIS data…..

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H1 QCD jet fit

Low sensitivity of fits to inclusive cross section to gluonPDF especially at large zIP →use jets to combined fits!

Largestdifference Largest

difference

JHEP 0710:042,2007

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Data 99/00

Eur.Phys.J.C52: 83 (2007)

Dijets in diffractive DIS, ZEUS

E*tjet1 > 5 GeV

The best agreement of data and NLO for H1 2006 fit B andMRW 2006

H1, ZEUS: for DIS dijets factorization holds…..

R=data/NLO(ZEUS LPS)

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pp

γ*p

Exporting DPDFs fromHERA to Tevatron…..

xIP integrated effective DPDFsfrom CDF single diff. dijets (run I)

Factorization broken by β-dependent factor ~ 10 !

If for HERA incl. datarapidity gap survival factor =1, at Tevatron ~0.1

Hadron-hadron collisions

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Photoproduction, γ⋆p, Q2→0

direct photoproduction (Q2≃0): photon directly involved in hard scattering xγ=1Expect: gap survival probablity= 1

resolved photoproduction (Q2≃0): photon fluctuates into hadronic

system, which takes part in hadronic scattering,dominant at Q2≃0

xγ<1Expect: gap survival probability < 1Theory (Kaidalov at al) ~0.34

hadronsz

jetszOBS

pE

pExx

)(

)(

xγ - fraction of photon’s momentum in hard subprocess

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H1: Etjet1 > 5 GeV suppression of factor ~0.5ZEUS: Etjet1 > 7.5 GeV weak (if any) suppression (0.6-0.9)

Neither collaboration sees difference between the resolved and direct regions, in contrast to theory!

Possible explanation of differences between H1 and ZEUS (DIS 2007)Different phase space of both analyses ……..?

One year ago….

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Et dependence of suppression?H1 ZEUS

H1 and ZEUS observe the data have harder Et slope than NLO

From the DIS 2008 talk of W.Slomiński,ZEUS results

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Double ratios of γ⋆p & DIS (data/NLO)

Very useful – full or partial cancellation of many uncertainties (energy scales for data, DPDFs used…etc ).

Etjet1>5 GeV

There is no clear W dependence butwhat about Etjet1

dependence??

H1 collab. Eur.Phys.J,C51 (2007),549

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H1 - ratio photoproduction/DIS

Double ratio of Data/NLO for photoproduction and DIS→ uncertainties of diffractive PDFs cancel!Figure derived from published results

Double ratio is within errors Et dependent!

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New H1 analysis – data 99/00

Tagged photoproduction, luminosity 3x larger than for 97 diffractive events found by Large Rapidity Gap method

(LRG)

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Two cut scenariosTo crosscheck previous H1 results To approach closest to ZEUS cuts

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Theoretical predictions

2 programs for NLO calculations, 3 sets of DPDFs:

Frixione/Ridolfi → H1 2006 Fit A

H1 2006 Fit B H1 2007 Fit Jets →

Kramer/Klasen → H1 2006 Fit B

The aim was to estimate the effect on DPDF used and to crosscheckNLO programs

inclusive measurement

DIS dijets

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Integrated survival probabilities (ISP)

No difference in survival probabilitiesfor resolved and direct regions of xγ,

like in previous H1 and ZEUS analyses

Within errors no difference in ISPusing different DPDFs

Lower Et cut scenario

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Lower Et cut scenario

Another hint of Et harder slope for datathan NLO

Hadronization corrections

δhadr=MC(hadr)/MC(parton)

ZEUS – W.Slominski

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Higher Et cut scenario

Now much more „direct-like“ events thanin low Et analysis, peak at higher xγ

Integrated survival probabilities (ISP)

Larger ISP than for lower Et cut scenario → more close to ZEUS results!!!

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Higher Et cut scenario

Et dependence not excluded but cannotbe independently verified

ZEUS – W.Slomiński

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What DPDF is the „best“?

The most sensitivevariable → zIP

Fits 2006 A,B valid onlyto zIP =0.8

Fit 2007 jets to zIP = 0.9

Fits 2006 A and 2007 Jetsrepresent extremes.

Fit 2006 B is in the middle….

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Summary

• new H1 dijet photoproduction data – older H1 results confirmed – within errors is gap survival probability (GSP) ~ 0.5.

• in higher Et cut scenario (similar to ZEUS) GSP is ~ 0.6, more close to ZEUS results.

• hint that GSP is dependent on Et of the leading jet, for low Et jets seems to be suppression more significant.

• the evidence that GSP is not different for direct and resolved events remains (originally not expected )

• to understand better diffractive dijets in photoproduction is challenging!!!