04.06.2011

06.06.2011 Low x workshop, Santiago de Compostela 2011 104.06.2011 1

Radek Žlebčík Karel Černý Alice Valkárová

Charles University, Prague

Factorisation in diffractive photoproduction at HERA

Low x workshop, Santiago de Compostela, 2011

06.06.2011 Low x workshop, Santiago de Compostela 2011 22

22

22

IP WQ

MQ

pq

ppqx X

22

2

XIP MQ

Q

x

x

W

´

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

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

My =mp proton stays intact, needsdetector setup to detect protons

My › mp proton dissociates, contribution should be understood

p p‘

MX

Q2

e

e‘

xIP

06.06.2011 Low x workshop, Santiago de Compostela 2011 3Low x workshop, Santiago de Compostella, 2011

3

Two types of factorisation

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

_

* QxtxQxfXpp iIP

D

ipartoni

D

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

2 QxxftxftxQxf IPIPiIPpIPIP

Di

QCD factorisation holds for inclusive and non-inclusive processes if:• photon is point-like (Q2 is high enough)• higher twist corrections are negligible (Mx is high enough)QCD factorisation theoretically proven for DIS (Collins 1998)

It allows the extraction of DPDFs from the (DIS) data

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

i* → hard scattering QCD matrix element, perturbatively calculated, process dependent

H1 and ZEUS –QCD fits assuming Regge factorisation for DPDF

pomeron PDF1)(2/ ),( tIP

Bt

IPpIP xe

txf pomeron flux factor


Tests of QCD factorisation

What kind of final states?• processes with a hard scale • sensitive to gluons (gluons contribute by up to 80% to the DPDFs).

Dijets and D* in DIS, D* in photoproduction – factorisation holds (tested by H1 and ZEUS).

What about dijets in photoproduction?Similarity with hadron-hadron interactions..

06.06.2011 Low x workshop, Santiago de Compostela 2011 524.06.2010 5

Photoproduction, γp, Q2→0

direct photoproduction photon directly involved in hard scattering

xγ=1 (at parton level)

hadron –like component photon fluctuates into hadronic system, which takes part in hadronic scattering

xγ<0.2 (at parton level)point –like component of resolved

photon

dominates in the region of 0.2<xγ<1

xγ - fraction of photon’s momentum in hard subprocess

In LO!

resolved

direct

Similarity withhadron-hadroncollisions

06.06.2011 5


Exporting DPDFs from HERA to Tevatron does not work

Factorisation broken by β-dependent factor ~ 10, S2 ~ 0.1.

Factorisation in hadron-hadron collisions

S2=σ (data)σ (theory)

suppression factor

In 2010 new theoretical prediction by KKMR:(European Journal of Physics 66,373 (2010))Suppression 0.34 present only for hadronic part of photon PDF (xγ<0.2 ), for dominant point-like component suppression: quarks GRV 0.71(0.75) ET

jet1 >5 (7.5) GeV gluons GRV 0.53(0.58) ET

jet1 >5 (7.5) GeV

6Low x workshop, Santiago de Compostella, 2011

According to theoretical predictions still some suppression around 0.5-0.8 should exist!

Dijets in diffractive photoproduction


Dijets in photoproduction

Factorisation breaking observed by H1, two analyses,EPJC C51 (2007),549, – suppression ~ 0.5EPJ C68 (2010),381 – suppression ~ 0.6

not observed by ZEUS, Nucl.Phys. B381 (2010),1 – no suppression

Do we understand this difference?


Possible explanation….?

• the different phase space of both analyses – larger Et of jets in ZEUS analysis etc. (the influence of larger Et cut was however not fully confirmed – see EPJ C68 (2010),381 )

• in H1 and ZEUS different hadronisation corrections (estimated by MC RAPGAP and applied to NLO QCD calculations)

• old photon structure GRV – used however by both H1 and ZEUS

Here an attempt to crosscheck critical points….


DPDFs used

quark

quark

gluon

gluon

H1 fit B, H1 fit Jets, ZEUS fit SJ


H1 diff. cross sections and NLO


ZEUS diff. cross sections and NLO• Our calculations (ZEUS fit SJ), suppression ~ 1.1• ZEUS published (ZEUS fit SJ), suppression ~ 1.


Inclusive

Diffractive

Inclusive

Diffractive

Inclusive

Diffractive

21

GeV45

0.650.3

0.01GeV

2jet1

2jet1

22

<η<

<E

<y<

<Q

T

1.51.5

GeV6.57.5

0.850.2

1GeV

2jet1

2jet1

22

<η<

<E

<y<

<Q

T

0.8

1.6GeV

1GeV

0.032

<z

<M

<t

<x

IP

Y

IP

PY

IP

M=M

<t

<x21GeV

0.025

1.51.5

GeV6.57.5

0.650.3

0.01GeV

2jet1

2jet1

22

<η<

<E

<y<

<Q

T

1.6GeV

1GeV

0.0252

<M

<t

<x

Y

IP

IPT x,E,η jetsjets

IPz,η jets

y,Q2

expansion

contraction

contraction

H1 – high PtPreliminary

H1

ZEUS

expansion MYexpansion IPz,ηy,,Q jets2

contraction YIPT M,η,x,E jetsjets

Extrapolation


Method of Extrapolation

H

1

Z

E

U

S

sy=Wy

Some events missing

WWyWy σ=σ+σM add

W

W5

W4

W3

W2

W1

y5

y4

y3

y2

y1

y σ=

σ

σ

σ

σ

σ

σ

σ

σ

σ

σ

=σ

H1cuts

ZEUScuts Vector of x-sections

in W bins which we want to

estimate

Vectorof measuredX-sectionsin y bins

Probability that event in yj bin will be registered in Wi (from MC fitted to the data)

WiyjijWy p=M

Wiyjp

1 Wiyjp

addWσ

Vector of x-sections of events which don't fulfill H1 cuts but agree with ZEUS (from MC RAPGAP)

Transition matrix


Inclusive

Diffractive

Inclusive

Diffractive

21

GeV45

0.650.3

0.01GeV

2jet1

2jet1

22

<η<

<E

<y<

<Q

T

0.8

1.6GeV

1GeV

0.032

<z

<M

<t

<x

IP

Y

IP

1.51.5

GeV6.57.5

0.650.3

0.01GeV

2jet1

2jet1

22

<η<

<E

<y<

<Q

T

1.6GeV

1GeV

0.0252

<M

<t

<x

Y

IP

IPT x,E,η jetsjets

IPz,η jetsexpansion

contraction

H1 – high PtPreliminary

H1

Test of method

See: K.Černý, talk in DIS08,London 2008


Results, H1 → H1 High Pt


Inclusive

Diffractive

Inclusive

Diffractive

21

GeV45

0.650.3

0.01GeV

2jet1

2jet1

22

<η<

<E

<y<

<Q

T

1.51.5

GeV6.57.5

0.850.2

1GeV

2jet1

2jet1

22

<η<

<E

<y<

<Q

T

0.8

1.6GeV

1GeV

0.032

<z

<M

<t

<x

IP

Y

IP

PY

IP

M=M

<t

<x21GeV

0.025

H1

ZEUS

expansion IPz,ηy,,Q jets2

contraction YIPT M,η,x,E jetsjets

Extrapolation


Results, H1 → ZEUS

Suppression of H1 data & ZEUS data is ~ 0.6


Conclusions

• NLO calculations (with 3 DPDFs) and hadronisation corrections checked for H1 and ZEUS

• Sensitivity to photon structure function checked

• H1 data extrapolated with the help of MC RAPGAP to ZEUS phase space

Data of H1 and ZEUS are different in theidentical (ZEUS) phase space….

04.06.2011

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