diffractive dijets atdiffractive dijets at hera · diffractive event selection 1) proton...

Diffractive dijets atDiffractive dijets at HERA

Representing H1 and ZEUS experiments

HERA/LHC workshop May 2008

Alice ValkárováCh l U i it P

1

Charles University, Prague

HERA collider experiments• 27.5 GeV electrons/positrons on 920 GeV protons →√s=318 GeV• two experiments: H1 and ZEUS

HERA I 16 b 1 120 b 1• HERA I: 16 pb-1 e-p, 120 pb–1 e+p• HERA II: ∼ 550 pb-1, ∼ 40% polarisation of e+,e-• closed July 2007, still lot of excellent data to analyse……

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DIS: Probe structure of proton → F2

Diffractive DIS: Probe structure of

HERA/LHC workshop May 2008

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Diffractive DIS: Probe structure of color singlet exchange → F2

D

Diffraction and diffraction kinematics

´

Two classes of diffractive events:

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

2HERA: ~10% of low-x DIS events are diffractive

Q >>0 → deep inelastic scattering (DIS)

( )22

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IP WQMQ

pqppqx X

++≈

⋅′−⋅=

βW momentum fraction of color singlet exchange

22

2

XIP MQQ

xx

+≈=β

W m m m f f g g

XQfraction of exchange momentum, coupling to γ*

2, )( ppt −=t

4-momentum transferd

My

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)( ppt squared

Diffractive Event Selection1) 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 xIPsmall acceptance low statistics ☠small acceptance low statistics ☠

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

e p

t not measured, some p-diss background ☠

3) Mx method, ZEUS:Diffractive vs non diffractive: exponential

)ln(tan- η 2θ=

Diffractive vs non-diffractive: exponential fall off vs constant distribution in ln Mx

2

Some p-diss contribution ☠2lnMBdN

diff. non-diff

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ln2ln

xMB

x

CeDMd

dN +∝

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

H d tt i QCDHard scattering QCDmatrix element,perturbativelycalculated, process dependent

Universal diffractive partondensities, identical for all

processesprocesses

),(),,,()( 2*2* QxtxQxfXpp iIP

D

ipartoni

D γσγσ ⋅=→ ∑_ iparton

Dif

*

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

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

Proton vertex (Regge) factorizationProton vertex (Regge) factorizationAdditional assumption – there is no proof!

Bte

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

2 QxxftxftxQxf IPIP

iIPpIPIPD

i =⋅= β

1)(2/ ),( −= tIP

Bt

IPpIP xetxf α pomeron flux factor pomeron PDF

Exctracted from inclusive diffraction!Fits 2006 A and BFits 2006 A and B

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H1 2006 fits -resultsH1 2006 fits resultsz (zIP)- longitudinal momentum fraction of gluon rel. to

Two fits, A and BFi B l i d

colorless exchange

Fit B: gluon parameterised as aconstant at starting scale

• quarks are very stable• gluons carries ~75% ofgluons carries ~75% ofpomeron momentum

• gluons similar at low z• no sensitivity to gluon

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no sens t v ty to gluonat high z

Dijets in diffractive DIS, H14 < Q2 < 80 GeV2

0.1 < y < 0.7x 0 03xIIP < 0.03

P*tjet1 > 5.5 GeVP* > 4 GeV

Data 99/00

P tjet2 > 4 GeV-3. < η*jets < 0.

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

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Dijets in diffractive DIS, H1j ff ,

For zIP < 0.4 NLO predictionsusing fits 2006 A and B agree with data very wellagree 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 toinclusive cross section to gluonPDF especially at large zIP →DF sp c a y at arg zIPuse jets to combined fits!

JHEP 0710:042,2007

Largestdifference Largest

differenceAA

B Ajets

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Dijets in diffractive DIS, ZEUS

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

Data 99/00E*tjet1 > 5 GeV

R=data/NLO(ZEUS LPS)

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

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11H1, ZEUS: for DIS dijets factorization holds…..

Hadron-hadron collisions

γ*p

Exporting DPDFs fromHERA to Tevatron…..

pp

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

Effect is known since 10 years!

Factorization broken by

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Factorization broken by β-dependent factor ~ 10 !

Photoproduction, γ⋆p, Q2→0

xγ - fraction of photon’s momentum in hard subprocess

jetszOBS

pEpE

xx)()( −

== ∑γγ

hadronszpE )( −

direct photoproduction (Q2≃0):photon directly involved in hard scattering

1

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

system, which takes part in hadronicscattering,dominant at Q2≃0

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xγ=1(at parton level)

g, Qxγ<1(at parton level)

Photoproduction as hadronic process

HERA resolved photoproduction

Secondary interactionsbetween spectatorsp

Typical models that describe suppression at Tevatron assume secondaryinteractions of spectators as the cause:

resolved contribution expected to be suppressed by factor 0 34

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

Photoproduction, γ⋆p, Q2 ⋍ 0

xγ - fraction of photon’s momentum in hard subprocessExpectation: Gap Survival Expectation: Gap Survival

P b bili 1

jetszOBS

pEpE

xx)()( −

== ∑γγ

p pProbability =1 Probability < 1

hadronszpE )( −

direct photoproduction (Q2≃0):photon directly involved in hard scattering

1

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

system, which takes part in hadronicscattering,dominant at Q2≃0

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xγ=1(at parton level)

g, Qxγ<1(at parton level)

One year ago….

H1 E 5 G V i f f t 0 5H1: 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 ?

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Different phase space of both analyses ……..?

Et dependence of suppression?Et dependence of suppression?H1 ZEUS

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

HERA/LHC workshop May 2008

17H1 and ZEUS observe the data have harder Et slope than NLO

Double ratios of γ⋆p & DIS(d t /NLO)(data/NLO)

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

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

There is no clear W dependencebut

what about Etj t1 dependence??what about Etjet1 dependence??

Etjet1>5 GeV

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H1 – double ratio, Et dependenceH1 double ratio, Et dependence Double ratio of Data/NLO for h d i d DISphotoproduction and DIS

→

Figure extracted from published results

Double ratio is within errors Et dependent!

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New H1 analysis – data 99/00New H1 analysis data 99/00Tagged photoproduction, luminosity 3x larger than for 97diffractive events found by Large Rapidity Gap method (LRG)diffractive events found by Large Rapidity Gap method (LRG)

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

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Theoretical predictionsTheoretical predictions

2 programs for NLO calculations, 3 sets of DPDFs:2 programs for NLO calculations, 3 sets of DPDFs

Frixione/Ridolfi → H1 2006 Fit Ainclusive measurement

H1 2006 Fit BH1 2007 Fit Jets →

inclusive measurement

DIS dijets

Kramer/Klasen → H1 2006 Fit B

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

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Lower Et cut scenario

Integrated survival probabilities (ISP)

Within errors no difference in ISPusing different DPDFs

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No difference in survival probabilitiesfor resolved and direct regions of xγ,like in previous H1 and ZEUS analyses

Lower Et cut scenario

Another hint of Et harder slope for datah NLOthan NLO

Hadronization corrections

δ =MC(hadr)/MC(parton)δhadr=MC(hadr)/MC(parton)

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Higher Et cut scenario

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

Integrated survival probabilities (ISP)Integrated survival probabilities (ISP)

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

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more close to ZEUS results!!!

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 =0.8Fit 2007 jets to z P = 0 9Fit 2007 jets to zIP = 0.9

Poorly constraint DPDFsgive large uncertainties atgive large uncertainties atlarge zIP.Fits 2006 A and 2007 Jetsrepresent extremes.p

Fit 2006 B is in the middle….

L E t i

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Lower Et cut scenario Higher Et cut scenario

Summary1 d h d d ld 1 l f d• new H1 dijet photoproduction data – older H1 results confirmed

– within errors is gap survival probability (GSP) ~ 0.5.

i hi h E t s i (si il t ZEUS) GSP is 0 6• in higher Et cut scenario (similar to ZEUS) GSP is ~ 0.6, more close to ZEUS results.

• hint that GSP is dependent on E of the leading jet• 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 andthe evidence that GSP is not different for direct andresolved events remains (originally not expected )

• the problem is rather the theoretical uncertainties than pthe lack of experimental results! NNLO????

• important for LHC predictions….(depend on DPDFs from HERA,d G P)

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and GSP)