Alice Valk

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H1 measurements of the structureof diffraction
and tests of factorisation

• Alice Valkárová
• on behalf of H1 collaboration
• LOW x meeting 2005, Sinaia

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HERA experiments and diffraction

HERA 10 of low-x DIS events are diffractive
? study QCD structure of high energy diffraction
with virtual photon
DIS Probe structure of proton ? F2
e
e

Diffractive DIS Probe structure of color singlet
exchange ? F2D
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Diffraction kinematics
Large rapidity gap between leading proton p and X
?max

W

• Momentum fraction of proton carried by color
  singlet exchange
• Momentum fraction of color singlet carried by
  struck quark

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QCD factorisation
get PDFs from inclusive diffraction ? predict
cross sections for exclusive diffraction
inclusive
dijet
hard scattering QCD matrix element,
perturbatively calculated, process dependent

• proven for DIS (J.Collins (1998))
• not proven for photoproduction!

Universal diffractive parton densities identical
for all processes
universal hard scattering cross section (same as
in inclusive DIS) diffractive parton distribution
functions ? obey DGLAP universal for diffractive
ep DIS (inclusive, di-jets, charm)
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Results from inclusive diffraction
Q2
ß
Regge factorisation is an additional assumption,
there is no PROOF!!

pomeron flux factor
pomeron PDF
sdiff flux(xP) object (ß,Q2)
Reduced cross section from inclusive diffractive
data

• get diffractive PDFs from a NLO (LO)
• DGLAP QCD Fit to inclusive data from
• 6.5 GeV2 to 120 GeV2
• extrapolation of the Fit
• to lower Q2
• to higher Q2
• gives a reasonably good description of
• inclusive data from 3.5 GeV2 1600 GeV2

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Diffractive Parton Densities

• determined from NLO QCD
• analysis of diffractive structure
• function
• more sensitive to quarks
• gluons from scaling violation,
• poorer constraint
• gluon carries about 75 of pome-
• ron momentum
• large uncertainty at large zP

Assuming factorisation holds, the jet and HQ
cross sections give better constraint on the
gluon density
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Jet and HQ production
Hard scale is ET of the jet or HQ mass
Direct access to gluon density
Can reconstruct zP in dijet events

• tests of universality of PDFs
• (QCD factorisation)
• test of DGLAP evolution

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Charm cross section (DIS)
Good agreement within experimental
theoretical uncertainties.
Factorisation holds !
NLO calculations with PDFs from inclusive
diffraction
NLO calculations HVQDIS (Harris Smith)
Good description of diffractive D production in
DIS (2GeV2 ltQ2lt100 GeV2)
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Dijets in DIS
NLO calculations diffractive extension of
DISENT CataniSeymour (Nucl.Phys.B485 (1997)
29), interfaced to diffr.PDFs of H1 Hadronisation
corrections RAPGAP MC
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Dijets in DIS

• NLO corrections to LO
• are significant factor 1.9
• excess at high x? is
• kinematically connected
• with the lack of events with
• ?lab of jets lt -0.4 in
• comparison with NLO

Factorisation holds!
Good agreement with NLO within exp.theor. uncerta
inties
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Exporting PDFs from HERA to the Tevatron.........
At Tevatron HERA PDFs do not work.????
Dijet cross section factor 5-10 lower than the
QCD calculation using HERA PDFs
Breakdown of factorisation!
?
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Direct and resolved processes at HERA
x? - fraction of photons momentum in
hard subprocess

• DIS (Q2gt5GeV2) and direct photoproduction (Q2?0)
• photon directly involved in hard scattering
• x?1

?
unsuppressed!

• Resolved photoproduction
• photon fluctuates into hadronic system, which
• takes part in hadronic scattering
• dominant at Q2?0
• x?lt1

?
suppressed!
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Photoproduction as hadronic process
HERA resolved photoproduction
Secondary interactions between spectators
Jets in photoproduction thought to be ideal
testing ground for rescattering
Typical models that describe suppression at
Tevatron assume secondary interactions of
spectators as the cause
resolved contribution expected to be suppressed
by factor 0.34 (Kaidalov,Khoze,Martin,RyskinPhys
.Lett.B567 (2003),61)
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Dijets in photoproduction
The same kinematical region as for DIS

• NLO overestimates the cross section by
• factor 2
• both direct and resolved are suppressed
• RAPGAP LO good description

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Dijets in photoproduction
If only resolved part is suppressed (by factor
0.34, according to Kaidalov et al.) ? data are
not described by NLO
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Ratiodata over NLO prediction

• no suppression observed for DIS
• overall suppression factor of
• about 2 observed for both
• resolved and direct
• components in photoproduction
• suppression is independent of the
• cms energy W

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Summary

• Dijets in DIS D cross section
  
  

• agree with the NLO prediction with the H1 2002
  diffractive pDFs
• factorisation holds (assuming PDF is correct)

• Dijets in photoproduction

• to investigate the puzzle of disagreement of
  HERA/Tevatron data
• (expectation resolved will be suppressed and
  direct not)
• data are half of NLO prediction both resolved
  and direct are
• suppressed ? conflict with the theoretical
  expectation

• More ideas?

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LPS proton vs Rapidity Gap

• Large rapidity gap selection
• MYlt1.6 GeV and tlt1 GeV2
• LPS proton selection MY mp
• extrapolated to tlt1 GeV2
• ?Good agreement between two methods and two
  experiments
• ?Data well described by
• H1 QCD fit to LRG data
• ZEUS Mx data should be scaled by 0.69
  to account for p-diss

M.Kapishin, Inclusive diffraction at HERA
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H1 and ZEUS