Jets in diffraction and factorization at HERA

On behalf of H1 and ZEUS collaborations

Alice Valkárová Charles University, Prague

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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: ∼ 500 pb -1 , ∼ 40% polarisation of e+,e closed July 2007, still lot of excellent data to analyse…… DIS: Probe structure of proton → F 2 e Diffractive DIS : Probe structure of color singlet exchange → F 2 D Low x workshop, Kolimpari 2 8.7.2008

Diffraction and diffraction kinematics

Two classes of diffractive events: Q 2 ~0 → Q 2 >>0 → photoproduction ´ HERA: ~ 10% of low-x DIS events are diffractive

t M y

W

x

IP



q

 

p q

 

p p

  

Q

2

Q

2

 

M X W

2 2

momentum fraction of color singlet exchange  

x x IP



Q

2

Q



2

M X

2

fraction of exchange momentum, coupling to γ*

t



(

p



p

, ) 2

4-momentum transfer squared Low x workshop, Kolimpari 8.7.2008

3

Diffractive Event Selection

1) Proton Spectrometers: ZEUS : LPS (1993-2000) H1 : FPS (1995-200 7 ),VFPS (2004-0 7 ) t measurement access to high x IP range free of p-dissociation background at low x IP small acceptance  low statistics ☠

e

2) Large Rapidity Gap, H1, ZEUS: Require no activity beyond η max t not measured, some p-diss background ☠

η



ln(tan θ 2 )

3) M x method, ZEUS : Diffractive vs non-diffractive: exponential fall off vs constant distribution in ln M x 2 Some p-diss contribution ☠ diff. non-diff

dN



d

ln

M x

2

D



Ce B

ln

M

2

x

Low x workshop, Kolimpari 8.7.2008

p

4

Dijets in diffractive ep scattering

Pointlike photon

LO

Resolved photon x γ - fraction of photon’s momentum in hard subprocess

x

 

x



OBS

 

(

E

(

E



p z



p z

)

jets

)

hadrons

DIS, direct PHP

photon directly involved in hard scattering

x γ =1

(at parton level), not exactly true for measured

x γ

8.7.2008

resolved PHP

photon fluctuates into hadronic system, which takes part in hadronic scattering,dominant at Q 2 ≃ 0

x γ <1

(at parton level) Low x workshop, Kolimpari 5

QCD factorization

Get PDF from inclusive diffraction predict cross sections for exclusive diffraction Hard scattering QCD matrix element,perturbatively calculated, process dependent Universal diffractive parton densities, identical for all processes 

D

(  *

p



Xp

)  

parton

_

f i i D

(

x

,

Q

2 ,

x IP

,

t

)    *

i

(

x

,

Q

2 )

f i D

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

i

→ universal hard scattering cross section (same as in inclusive DIS) Low x workshop, Kolimpari 8.7.2008

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Dijets in diffractive DIS, H1

4 < Q 2 < 80 GeV 2 0.1 < y < 0.7

x IIP < 0.03

Data 99/00 P* tjet1 P* tjet2 -3. < η* > 5.5 GeV > 4 GeV jets < 0.

8.7.2008

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

Data agree with NLO prediction, NLO with fit B is more close to data published, JHEP,0710:042,(2007) Low x workshop, Kolimpari 7

Dijets in diffractive DIS, H1

For z IP < 0.4 NLO predictions using fits 2006 A and B agree with data very well Combined QCD fit for inclusive and dijet DIS data…..

8.7.2008

Low x workshop, Kolimpari

A B jets

Largest difference 8

8.7.2008

Dijets in diffractive DIS, ZEUS

Eur.Phys.J.C52: 83 (2007) E* tjet1 > 5 GeV 5 < Q 2 < 100 GeV 2 Data 99/00 R=data/NLO(ZEUS LPS) Conclusions: the best agreement of data and NLO for H1 2006 fit B and MRW 2006 9

Photoproduction as hadronic process

Factorization broken by β dependent factor ~ 10 HERA resolved photoproduction Secondary interactions between spectators Rescattering leads to factorization breaking and rapidity gap fill up

suppression of cross section ~ 1 – (rapidity survival probability)

8.7.2008

resolved contribution expected to be suppressed by factor 0.34 (Kaidalov,Khoze,Martin,Ryskin:Phys.Lett.B567 (2003),61) Low x workshop, Kolimpari 10

Dijets in photoproduction, one year ago….

H1: E tjet1 > 5 GeV suppression of factor ~0.5

ZEUS: E tjet1 > 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 ……..? 8.7.2008

Low x workshop, Kolimpari 11

E

t

dependence of suppression?

From the DIS 2008 talk of W.Slomiński, ZEUS results Double ratio NLO/data for photoproduction and DIS Cancelations of several uncertainties (DPDFs for example) Figure extracted from the published results of H1 H1 and ZEUS observe the data have harder E t slope than NLO 8.7.2008

Low x workshop, Kolimpari 12

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 13

Two cut scenarios

To crosscheck previous H1 results To approach closest to ZEUS cuts 8.7.2008

Low x workshop, Kolimpari 14

8.7.2008

Lower E

t

cut scenario

2 programs for NLO calculations, 3 sets of DPDFs: Frixione/Ridolfi → H1 2006 Fit A H1 2006 Fit B H1 2006 Fit Jets Kramer/Klasen → Michael K.) H1 2006 Fit B (thanks to Hadronization corrections δ hadr =MC(hadr)/MC(parton) No difference in survival probabilities for resolved and direct regions of x γ, like in previous H1 and ZEUS analyses Low x workshop, Kolimpari 15

Lower E

t

cut scenario

Integrated survival probabilities (ISP) 8.7.2008

Within errors no difference in ISP using different DPDFs Another hint of E t than NLO harder slope for data Low x workshop, Kolimpari 16

8.7.2008

Higher E

t

cut scenario

Now much more „direct-like“ events than in low E t analysis, peak at higher x γ Integrated survival probabilities (ISP) Larger ISP than for lower E t cut scenario → more close to ZEUS results!!!

Low x workshop, Kolimpari 17

8.7.2008

Higher E

t

cut scenario

E t dependence not excluded but cannot be independently verified ZEUS – W.Slomiński Low x workshop, Kolimpari 18

Do we need global suppression?

M.Klasen,G.Kramer: Review of factorization breaking in diffractive photoproduction of dijets, DESY 08-074, Modern Phys.Lett.A,2008, an attempt to interprete ZEUS and H1 results with NLO resolved only suppression R=0.31 (0.29) for H1 and R=0.53 (0.45) for ZEUS published analyses H1, 2007 ZEUS, 2007 8.7.2008

Low x workshop, Kolimpari 19

8.7.2008

Global and resolved only suppression…

H1 lower Et cut scenario Global suppression 0.53

Data H1 preliminary, suppression of NLO resolved component 0.3 (see KK paper) Much worse agreement than for global suppression Low x workshop, Kolimpari 20

8.7.2008

Global and resolved only suppression…

H1 lower Et cut scenario Global suppression 0.53

Data H1 preliminary, suppression of NLO resolved component 0.3, (see KK paper) No E for x t dependence but prize is worse agreement data&NLO γ and also for other distributions!!!

Low x workshop, Kolimpari 21

Summary

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

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

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

• the evidence that GSP is resolved events

remains

not different for direct and (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) Low x workshop, Kolimpari 8.7.2008

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What DPDF is the „best“?

Lower E t cut scenario The most sensitive variable → z IP Fits 2006 A,B valid only to z IP = 0.8

Fit 2007 jets to z IP = 0.9

Poorly constraint DPDFs give large uncertainties at large z IP.

Fits 2006 A and 2007 Jets represent extremes.

Fit 2006 B is in the middle….

Higher E t cut scenario Low x workshop, Kolimpari 8.7.2008

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