Anna Mastroberardino

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Diffraction at HERA

• Anna Mastroberardino
• Calabria University
• On behalf of the
• H1 ZEUS Collaborations

HSQCD 2004 St. Petersburg, Russia 18 22 May 2004
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Outline

• Introduction to diffraction
• Diffractive structure function of the proton
• QCD fits of diffractive data
• Test of QCD factorization with jets and charm
• Exclusive vector mesons
• Summary

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What is diffraction?

Standard DIS in a frame in which the proton is
very fast (Breit frame)
The struck quark carries fraction x ? Q2/W2 of
the proton momentum W photon-proton centre of
mass energy
Diffraction exchange of colour singlet (IP)
producing a rapidity GAP in the particle flow
The pomeron carries fraction xIP of the initial
proton momentum The struck parton carries
fraction ß of the Pomeron momentum
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Why diffraction ?
DIS probes the partonic structure of the proton
Diffractive structure function
Diffractive cross section
Diffractive DIS probes the partonic structure of
colour singlet exchange

HERA has opened up the small x domain
920 GeV proton 27.5 GeV electron
10 of low-x DIS diffractive at HERA
What role does it play?
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Selection methods

• Two systems X and Y well separated in phase
• space with low masses MX ,MY ltlt W
• Y proton or p-dissociation
• carries most of the hadronic energy
• X vector meson, photon or
  photon-dissociation

Diffractive events are characterized by
scattered proton almost intact
no forward energy deposition
flat vs ln MX2 distribution
ln MX2
-2 0 2 4 6 8
Diffractive peak
Large Rapidity Gap
MX Method
Proton Tagging
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Factorization in Diffractive DIS

• QCD factorization for diffractive DIS holds
• (Collins, Bereira Soper, Trentadue
  Veneziano)

universal partonic cross section (same as in
inclusive DIS)
diffractive parton distribution function evolve
according to DGLAP universal for diffractive ep
DIS (inclusive, dijets, charm)

• If in addition postulate Regge factorization
  (Ingelman Schlein)

M.Kapishin, F2D measurements
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New results from ZEUS
Proton tagging method
MX method
MN lt 2.3 GeV
Transition from very low Q2 to DIS (0.03 ltQ2lt100
GeV2)
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Recent results from H1
Integrate over t ?

• high precision measurement of
• b and Q2 dependences
• QCD fit
• (DGLAP evolution of diffractive pdfs)
• (coming later)

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Measurement of b Q2 dependences

• Regge factorization holds for xIPlt 0.01
• Weak b dependence looks like a photon more than
  a proton
• Scaling violations positive up to large b
  large gluon contribution
• DGLAP evolution based fit describes the data

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H1 NLO QCD fit diffractive PDFs
Integrated fraction of exchanged momentum
carried by gluons (75 ? 15)
Diffractive data fitted in similar way to proton
F2 data

• Parametrize Flavour Singlet (quarks
  antiquarks)
• and gluons at Q2 3 GeV2
• Evolve according to NLO DGLAP and fit
• Determine quark sea and gluon distribution

Diffractive interactions gluon dominated
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ZEUS NLO QCD fit to F2D and charm
(LPS)

• xIP lt0.01
• QCDNUM
• Regge factorisation assumption
  possible for this small data set
• DL flux
• initial scale Q22 GeV2
• zf(z)(a1a2za3z2)(1-x)a4
• other PDFs parametrisation tried
• Thorne-Robert variable-flavour-
  number-scheme

• QCD fit describes data
• fractional gluon momentum
• shape of pdfs not well constrained

F2D(3)cc from DESY-03-094
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Factorization in Diffractive DIS experimental
test
universal partonic cross section (same as in
inclusive DIS)
diffractive parton distribution function evolve
according to DGLAP universal for diffractive ep
DIS (inclusive, dijets, charm)

• If QCD factorization holds diffractive parton
  densities are universal
• - Test use diffractive pdfs obtained so far from
  inclusive data to
• predict other final state cross
  sections

• diffractive DIS ?
• hadron hadron scattering?

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A test of QCD factorization jets and charm (H1)
Use results of NLO QCD fit to predict the rate
of diffractive production of dijets and charm in
DIS

• NLO calculations based on H1 pdfs describe data
  well
• QCD factorization in DDIS holds

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Factorization in Diffractive DIS experimental
test

• diffractive DIS ?

Diffractive structure function of antiproton
It holds

• hadron-hadron scattering ?

Factorization not expected to work - Indeed it
does not

• diffractive dijets at the Tevatron
• suppression by a factor of 10

factorization breaking

• understood in terms of (soft) rescattering
  
• corrections of the spectator partons
• (Kaidalov, Khoze, Martin, Ryskin)
• But several other approaches
• also a suppression of resolved g
• processes, supposed to be similar to pp ?

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Diffractive dijets in photoproduction
Real photon (Q2 0) can develop hadronic
structure
photoproduction similar to
hadron-hadron interaction

• Xg partonic momentum for dijet production

• photon remnant energy 1 - xg

• LO comparison no evidence for a suppression
• of resolved with respect to direct
• NLO comparison ?

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Diffractive dijets in photoproduction
NLO calculations compared to preliminary H1 data
(Klasen and Kramer, DESY 04-011)

• NLO comparison agreement between
• data and MC found if resolved
• contribution suppressed by a factor
• of 0.34
• rate of suppression expected
• from theoretical models

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Vector Meson production
(JPC1--) r, f, J/y,U,...
W

• Exclusive VM production calculable in pQCD
• NLO calculation available for J/Y
  photoproduction
• Sensitivity to gluons in proton

2-gluon exchange
probability of finding 2 gluons in the proton
cross section
rise with W increasing with hard scale
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Vector Meson production
Small MV (MV2 ? 1 GeV2) Incoming dipole behaves
like a normal-size hadron. Flat s vs W
reflects flat gluon distribution for Q2 ? 0
Soft regime
Large MV Fast growth of s with W reflects
growth of gluon distribution with decreasing x
Hard regime
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Exclusive J/Y Meson production
sL/sT vs Q2
0ltQ2lt100 GeV2
Pomeron trajectory

• - pQCD models describe data
• strong sensitivity to (generalized) gluon
• need NLO to constrain gluon density

aIP(t) not consistent with soft diffractive
measurement
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Summary
New high precision HERA data have improved our
understanding of diffraction
pQCD

• Diffractive processes are dominated by gluons

• Regge factorization works to a good approximation

Non perturbative phenomenology

• Diffractive pdfs are universal within HERA
• - QCD factorization holds in
  diffractive DIS

- On the way to understanding the large breaking
of factorization at Tevatron soft
re-scattering

• Vector mesons steep W dependence
• - Pure pQCD approach successful

Need to discriminate models
HERAII a lot of more data coming