1. Diffraction at HERA Alice Valkárová Charles University, Prague
Meeting Yerevan 2010
AFP meeting, Yerevan 2010

2. Content factorisation and DPDFs factorisation tests
HERA and diffraction
factorisation and DPDFs
factorisation tests
longitudinal part of DPDF
forward jets in diffraction
AFP meeting, Yerevan 2010

3. 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
closed July 2007, still lot of excellent data to analyse……
DIS: Probe structure of proton → F2
AFP meeting, Yerevan 2010

4. H1 data Lower energies of the proton, special runs to establish
longitudinal part of PDF and DPDF.
AFP meeting, Yerevan 2010

5. H1 detector
AFP meeting, Yerevan 2010

6. Deep inelastic scattering: Neutral Current
AFP meeting, Yerevan 2010

7. Diffraction 10% of DIS events at HERA are of diffractive type
colour singlet exchange – rapidity gap
AFP meeting, Yerevan 2010

8. Introduction – why to study diffraction?
To understand the nature of diffraction, high energy limit
of QCD.
Diffraction is a significant fraction of the inclusive cross section.
Can pQCD be used to describe diffraction?
Can we think of diffraction in terms of a factorisable structure
function and a hard scattering process (QCD factorisation)?
If so, what are the parton distributions (DPDFs) of the structure
functions….
Practical motivation: to study factorisation properties of diffraction –
try to transport to hh scattering (e.g.predict diffractive Higgs
production at LHC)
AFP meeting, Yerevan 2010

9. Diffraction and diffraction kinematics
Two kinematic regions of diffractive events:
Q2~0 → photoproduction
Q2>>0 → deep inelastic scattering (DIS) e‘ W Q2 e MX
xIP p p‘
momentum fraction of color singlet exchange My t
My =mp proton stays intact, needs
detector setup to detect protons
My › mp proton dissociates,
contribution should be understood
fraction of exchange momentum, coupling to γ
4-momentum transfer
squared
AFP meeting, Yerevan 2010
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10. Methods of diffractive ev.selection
Proton spectrometers
ZEUS: LPS ( )
H1: FPS ( )
VFPS ( )
t measurement
access to high xIP range
free of p-dissociation background at low xIP
small acceptance  low statistics ☠
Large Rapidity Gap, H1, ZEUS:
require no activity beyond ηmax
t not measured,
very good acceptance at low xIP
p-diss background about 20% ☠
Different systematics – non-trivial to compare!
Low x workshop, Kavala 2010
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11. Diffractive cross section
at low y
→ diffractive reduced cross section if
Longitudinal part of diffractive
structure funcion
Integrate over t when proton is not measured
→ σRD(3)(β,Q2,xP)
AFP meeting, Yerevan 2010

12. Two types of factorisation
QCD factorisation holds for inclusive and non-inclusive processes:
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)
→ DPDFs – obey DGLAP, universal for diff. ep DIS (inclusive,dijet,charm)
→ universal hard scattering cross section (same as in inclusive DIS)
It allows the extraction of DPDFs from the (DIS) data
H1 and ZEUS –QCD fits assuming Regge factorisation for DPDF
conjecture, e.g. Resolved
Pomeron Model by
Ingelman, Schlein
pomeron flux factor
pomeron PDF
Low x workshop, Kavala 2010
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13. QCD and diffraction 2 steps process:
parametrise the IP flux (xIP,t) & factorise it
(hypothesis)
fit diffractive PDfs (β,Q2)
AFP meeting, Yerevan 2010

14. f(xIP) from data
AFP meeting, Yerevan 2010

15. Soft pomeron, Regge model
Regge model: analytic model of
HADRONIC scattering
Exchange of collective states:
linear trajectories in the spin-
energy (α,t) plane,
σpp IP IR
αj(t)= αj(0)+α´j(t) (j=IR,IP,..)
In γ*p→XY virtual photon resolves
structure of exchanged object
dominant contribution looks similar
to soft pomeron,
we can extract Regge trajectory
AFP meeting, Yerevan 2010

16. xIP dependence for two t values
Reduced cross section
measured for the first time at
two t values
Low xIP and high β, decrease with
increasing xIP →IP-like behaviour
High xIP and low β, increase with
increasing xIP → IR-like behaviour
Lines – results of Regge fit
Regge fit, IP+IR
Not soft pomeron, different multi-pomeron absorption effects?
AFP meeting, Yerevan 2010

17. f(xIP) from data
AFP meeting, Yerevan 2010

18. Proton tagged data – t dependence
Fitting to e-bt → b ~6-7GeV-2
independent of β, Q2
Almost no xIP dependence →
AFP meeting, Yerevan 2010

19. Proton vertex factorisation
Regge fit in different Q2 bins, no strong evidence for αIP(0) dependence
Variables (xIP,t) describing proton vertex
factorise from those at photon vertex (Q2,β) to good approximation.
Q2 and β dependence interpreted in terms of
Diffractive Parton Densities (DPDF) →
measurement of partonic structure of exchange,
NLO QCD fits
AFP meeting, Yerevan 2010

20. Diffractive PDfs f(β,Q2)
AFP meeting, Yerevan 2010

21. DPDF functions H1 dijet DIS measurement: H1 fit Jets
Low sensitivity of fits to
inclusive cross section to gluon
PDF especially at large zIP →
use jets to combined fits!
H1 dijet DIS measurement:
H1 fit Jets
AFP meeting, Yerevan 2010

22. DPDF functions New QCD fit of ZEUS ZEUS fit SJ uses inclusive & DIS
dijet data
Not very different from
H1 fit jets…
AFP meeting, Yerevan 2010

23. Gluon fraction
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24. Comparison of H1 and ZEUS DPDFs
Differences in H1/ZEUS
fits:
Q2>8.5 GeV2 / Q2>5 GeV2
My< 1.6 GeV / My< mp,
scaling by 0.81
Reflects the consistency
between H1 and ZEUS
data
AFP meeting, Yerevan 2010

25. Comparison between methods
Are „rapidity gap“ and „forward proton“ methods compatible?
LRG selection contains about 20% events of proton diss.
no significant dependence on any variable
well controlled, precise measurements
ratio
ZEUS, LPS/LRG = 0.76  0.01  
0.03
0.08
0.02
0.05
Precise knowledge and corrections for proton dissociation background
- key point in H1- ZEUS data comparison
H1, LRG/FPS = 1.18  0.01 0.06 0.10
AFP meeting, Yerevan 2010

26. Comparison H1 FPS & ZEUS LPS
The cleanest comparison, all available data used by both collaborations,
H1 HERA II 20 times improved statistics, higher Q2
Fair agreement, normalisation uncertainty → LPS~10%, FPS ~6%
AFP meeting, Yerevan 2010

27. H1 & ZEUS inclusive diffraction,LRG
Q2 (GeV2)
Q2 (GeV2)
Good agreement between H1 and ZEUS in most of the phase space, (except low Q2), ZEUS scaled by 0.87, covered by normalisation uncertainty
AFP meeting, Yerevan 2010

28. Complete H1 data set Combined FPS,VFPS and LRG results. 10.1.2019
AFP meeting, Yerevan 2010

29. FLD extraction
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30. FLD extraction
The linear fits to Rosenbluths plots for two highest bins are
shown, allowing two new measurements at low Q2 to be made.
AFP meeting, Yerevan 2010

31. Longitudinal part of DPDF
A new measurement in low Q2 together with previously measured
FLD at medium Q2.
The data measured at the energy
of protons:
920,820,575 and 460 GeV.
The measurement is consistent
with the extrapolated fit B.
AFP meeting, Yerevan 2010

32. Photoproduction, γp, Q2→0 In LO! direct resolved
Similarity with
hadron-hadron
collisions
xγ - fraction of photon’s momentum
in hard subprocess
direct
resolved
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
direct photoproduction
photon directly involved in hard scattering
xγ=1 (at parton level)
AFP meeting, Yerevan 2010
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33. Tests of QCD factorisation
Basic strategy:
measure a particular diffractive final state
compare the measurement with NLO calculation
using DPDFs previously extracted
What kind of final states?
processes with a hard scale
sensitive to gluons (gluons contribute by up to 80%
to the DPDFs, mainly for high zIIP)
Dijets and D* in DIS/photoproduction are the
best candidates!
Low x workshop, Kavala 2010
AFP meeting, Yerevan 2010 33

34. Factorisation in hadron-hadron collisions
Exporting DPDFs from HERA to Tevatron does not work
S2=
σ (data)
σ (theory)
suppression factor
Factorisation broken by β-dependent factor ~ 10, S2 ~ 0.1.
Succesfully explained by terms of rescattering and absorption
(see Kaidalov,Khoze,Martin,Ryskin:Phys.Lett.B567 (2003),61)
KKMR predicted suppression factor for HERA resolved photoproduction
S2 ~ 0.34
In 2010 new theoretical prediction by KKMR:
(European Journal of Physics 66,373 (2010))
Suppression 0.34 is present only for hadronic part of photon PDF,
(dominant for xγ<0.2 ), for point-like component which is dominant
in our phase space – suppression: quarks GRV 0.75(0.71) ETjet1 >7.5 (5.)
gluons GRV 0.58(0.53) GeV
AFP meeting, Yerevan 2010
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35. What we learned from HERA data?
DIS dijets – factorisation theoretically predicted.
Both H1 and ZEUS confirmed experimentally and used
for QCD fits („H1 fit jets“, „ZEUS fit SJ“).
D* in DIS & photoproduction-
data within large errors not
in contradiction with factorisation
New ZEUS fits compared to
published DIS D* data.
(Nucl.Phys. B672 (2003),3.)
(Nucl.Phys. B831 (2010), 1)
Low x workshop, Kavala 2010
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36. What we learned from HERA data?
Photoproduction dijets – factorisation not predicted theoretically,
experimentally not fully understood…. different conclusions made
by H1 and ZEUS, H1 observed suppression about , ZEUS
negligible suppression (in different phase space, e.g.larger ET of
jets.)
ETjet1 > 7.5 GeV, ETjet2 > 5 GeV
Published ZEUS dijet photoproduction data
(Eur.Phys.J.C 55 (2008),177) compared to
NLO with „H1 fit Jets“ and „ZEUS fit SJ“
AFP meeting, Yerevan 2010

37. Dijet photoproduction
H1 data
DESY (2010)
Data compared to RAPGAP MC and NLO
GRV photon structure function
NLO calculations – Frixione/Ridolfi
3 sets of DPDFs
H fit B using inclusive data
H fit jets
ZEUS SJ fit
using DIS dijets
zIIP < 0.8
Low x workshop, Kavala 2010
AFP meeting, Yerevan 2010 37

38. Dijets in photoproductionSS
NLO with „H1 fit B“ larger cross section
than data. Shapes of distributions
are described. RAPGAP describes data
satisfactorily.
Low x workshop, Kavala 2010
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39. Shapes of distributions are described
succesfully, (with the exception of Etjet1),
global suppression of about 0.6 observed….
DPDF uncertainty
Scale variation uncertainty
Low x workshop, Kavala 2010
AFP meeting, Yerevan 2010 39

40. New ZEUS fit-comparison with old data
Published data: EPJ C55 (2008) 177
Very good description → no evidence for suppression
for ZEUS combined fit and H1 fit jets → ISP ~ 1…..
AFP meeting, Yerevan 2010

41. Double differential cross sections
Clear evidence for decrease
of resolved component with
increasing Et
The suppression in
resolved enriched region of xγ
may be Et dependent.
(suppression less for higher Et.)
ZEUS
ZEUS photoproduction dijets
Nucl.Phys. B831 (2010), 1
AFP meeting, Yerevan 2010
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42. Double differential cross sections
Shapes of distributions
described by NLO well.
Gap survival probabability
is insensitive to the
presence or nature of
remnant (either the photon
or diffractive exchange)
AFP meeting, Yerevan 2010

43. Ratio data/theory – 3 DPDFs
„H1 fit jets“ and „ZEUS
fit SJ“ give similar
agreement in shape.
„ZEUS fit SJ“ gives larger
prediction by about
15-20 % than „H1 fit B“.
Differences are covered
by theor. uncertainties.
Global suppression:
O.58 for „H1 fit B“
0.64 for „H1 fit jets“
0.70 for „ZEUS fit SJ“
Low x workshop, Kavala 2010
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44. Comparison with KKMR models
NLO calculations
Model KKMR 2003:
resolved part suppressed by 0.34.
Model KKMR 2010:
quarks suppressed by 0.71
gluons suppressed by 0.53
Model KKMR 2010 agrees with H1 data better than
model 2003 but shape description is still better
with global suppression.
Low x workshop, Kavala 2010
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45. Ratio diffractive to inclusive
Proposed by Kaidalov et al. Phys.Lett B567 (2003) 61
Full or partial cancellation of PDF uncertainties,
scales…..
Distribution of xγ sensitive to gap survival.
H1 - measured in same kinematic range
with same method as diffractive
cross sections
Acceptance corrections – PYTHIA
Problem → describes low Et inclusive data with inclusion of multiple interactions only,
large hadronisation corrections!
Such a low Et jets also not properly described by NLO –
see eg. H1 inclusive jet paper (EPJ C 129 (2003) 497)
AFP meeting, Yerevan 2010

46. Ratio diffractive to inclusive
Ep=820 GeV
Ep=930 GeV
zIP < 0.8
comparison to MC models - RAPGAP/PYTHIA
very different phase space for incl.& diffractive
large sensitivity to multiple interactions (MI) for inclusive dijets
better agreement of data ratio with PYTHIA MI
due to these facts - interpretation difficult
AFP meeting, Yerevan 2010

47. Diffractive forward jets
Jets measured in H1, protons in Forward Proton Spectrometer
Direct and accurate measurement
of diffractive variables,
free of proton diss.background,
small acceptance – low statistics
Data compared to NLOJET++
with H1 fit B,
hadronization uncertainties
about 8%
Cuts:
AFP meeting, Yerevan 2010

48. Comparison to previous analysis
Diffractive dijet analysis with
LRG – JHEP 0710:042
Data scaled by 1.23 (no proton
dissociation background)
Very good consistency, phase space
extension in xIP by factor 3
AFP meeting, Yerevan 2010

49. Diffractive forward jets
Within error, the NLO describes data well.
RAPGAP describes the shape but not normalisation
AFP meeting, Yerevan 2010

50. Conclusions-factorisation
DPDFs measured by H1 and ZEUS using assumption of Regge
factorisation (inclusive and DIS dijet measurement).
DIS dijets, D* in DIS & photoproduction – factorisation holds.
Dijets in photoproduction:
conclusions about factorisation using H1 and ZEUS data (with the
identical DPDFs) are different….
H1 – data suppressed by global factor (depending on DPDF).
ZEUS – compatible with no suppression
possible explanation
suppression is decreasing with increasing ET of the jets.
Longitudinal part of DPDF measured, measurement consistent with
H1 fit B.
Forward jets in DIS measured in new kinematic region, NLO describe
data well.
Low x workshop, Kavala 2010
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