1

Jets in diffraction and factorization at HERA

Alice Valkárová Charles University, Prague

On behalf of H1 and ZEUS collaborations

8.7.2008 Low x workshop, Kolimpari 2

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: ∼ 500 pb-1, ∼ 40% polarisation of e+,e-• closed July 2007, still lot of excellent data to analyse……

e

DIS: Probe structure of proton → F2

Diffractive DIS: Probe structure of

color singlet exchange → F2D

8.7.2008 Low x workshop, Kolimpari 3

22

22

IP WQ

MQ

pq

ppqx X

22

2

XIP MQ

Q

x

x

W

´

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

Diffraction and diffraction kinematics

momentum fraction of color singlet exchange

fraction of exchange momentum, coupling to γ*

2, )( ppt t

4-momentum transfersquared

My

Two classes of diffractive events:

Q2~0 → photoproductionQ2>>0 → deep inelastic scattering (DIS)

8.7.2008 Low x workshop, Kolimpari 4

Diffractive Event Selection

1) Proton Spectrometers: ZEUS: LPS (1993-2000) H1: FPS (1995-2007),VFPS (2004-07)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 Mx2

Some p-diss contribution ☠

2ln2ln

xMB

x

CeDMd

dN

diff. non-diff

)ln(tan- η 2θ

e p

8.7.2008 Low x workshop, Kolimpari 5

Dijets in diffractive ep scattering

photon directly involved in hard scattering

xγ=1 (at parton level),due to hadronization and resolutionnot exactly true for measured xγ

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

xγ<1(at parton level)

hadronsz

jetszOBS

pE

pExx

)(

)(

xγ - fraction of photon’s momentum in hard subprocess

LO

DIS, direct PHP resolved PHP

Pointlike photonResolved photon

8.7.2008 Low x workshop, Kolimpari 6

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!

8.7.2008 Low x workshop, Kolimpari 7

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

published, JHEP,0710:042,(2007)

8.7.2008 Low x workshop, Kolimpari 8

Dijets in diffractive DIS, H1 For zIP < 0.4 NLO predictions using fits 2006 A and

B agree with data very well

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

AB

jets

Largestdifference

8.7.2008 Low x workshop, Kolimpari 9

Data 99/00

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

Dijets in diffractive DIS, ZEUS

E*tjet1 > 5 GeV5 < Q2 < 100 GeV2

Conclusions: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)

8.7.2008 Low x workshop, Kolimpari 10

Photoproduction as hadronic process

resolved contribution expected to be suppressed by factor 0.34 (Kaidalov,Khoze,Martin,Ryskin:Phys.Lett.B567 (2003),61)

Rescattering leads to factorization breaking and rapidity gap fill up suppression of cross section ~ 1 – (rapidity survival probability)

HERA resolved photoproduction

Secondary interactionsbetween spectators

Jets in photoproductio

n thought to

be

ideal testin

g ground for re

scatterin

g

Factorization broken by β dependent factor ~ 10

8.7.2008 Low x workshop, Kolimpari 11

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 ……..?

Dijets in photoproduction, one year ago….

8.7.2008 Low x workshop, Kolimpari 12

Et dependence of suppression?

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

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

Double ratio NLO/data for photoproductionand DIS

Very useful !!!! Cancelations of several uncertainties (DPDFs for example)

Figure extracted from the publishedresults of H1

8.7.2008 Low x workshop, Kolimpari 13

New H1 analysis – data 99/00

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

(LRG)

8.7.2008 Low x workshop, Kolimpari 14

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

8.7.2008 Low x workshop, Kolimpari 15

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

like in previous H1 and ZEUS analyses

Lower Et cut scenario

Hadronization corrections δhadr=MC(hadr)/MC(parton)

Frixione/Ridolfi → H1 2006 Fit A H1 2006 Fit B H1 2006 Fit Jets

Kramer/Klasen → H1 2006 Fit B (thanks to Michael K.)

2 programs for NLO calculations, 3 sets of DPDFs:

8.7.2008 Low x workshop, Kolimpari 16

Lower Et cut scenario

Another hint of Et harder slope for datathan NLO

Integrated survival probabilities (ISP)

Within errors no difference in ISPusing different DPDFs

8.7.2008 Low x workshop, Kolimpari 17

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!!!

8.7.2008 Low x workshop, Kolimpari 18

Higher Et cut scenario

Et dependence not excluded but cannotbe independently verified

ZEUS – W.Slomiński

8.7.2008 Low x workshop, Kolimpari 19

Do we need global suppression?M.Klasen,G.Kramer: Review of factorization breaking in diffractivephotoproduction of dijets, DESY 08-074, Modern Phys.Lett.A,2008,

an attempt to interprete ZEUS and H1 results with NLO resolved onlysuppression R=0.31 (0.29) for H1 and R=0.53 (0.45) for ZEUS publish analyses

H1, 2007 ZEUS, 2007

8.7.2008 Low x workshop, Kolimpari 20

Global and resolved only suppression…

Global suppression 0.53

Data H1 preliminary,suppression of NLOresolved component0.3 (see KK paper)

H1 lower Et cut scenario

Much worse agreementthan for global suppression

8.7.2008 Low x workshop, Kolimpari 21

Global and resolved only suppression…

Global suppression 0.53

Data H1 preliminary,suppression of NLOresolved component 0.3, (see KK paper)

H1 lower Et cut scenario

No Et dependence butprize is worse agreement data&NLOfor xγ and also for other distributions!!!

8.7.2008 Low x workshop, Kolimpari 22

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 )

• the problem are rather the theoretical uncertainties than the lack of experimental results! (see huge scale uncertainties).

• important for LHC predictions….(depend on DPDFs from HERA, and GSP)

8.7.2008 Low x workshop, Kolimpari 23

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

Poorly constraint DPDFsgive large uncertainties atlarge zIP.

Fits 2006 A and 2007 Jetsrepresent extremes.

Fit 2006 B is in the middle….Lower Et cut scenario Higher Et cut scenario