Marcella Capua

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Inclusive Diffraction at ZEUS

• Marcella Capua
• Calabria University and INFN Cosenza (Italy)

on behalf of

• Introduction to diffraction
• Diffractive structure function and cross section
• Comparison with models
• QCD fits of diffractive data

Diffraction 2004 - International Workshop on
Diffraction in High-Energy Physics Cala Gonone,
Sardinia, Italy - September 18-23, 2004
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Diffraction at HERA
Standard Deep Inelastic Scattering
x fraction of protons momentum carried by
struck quark ? Q2/W2 W photon-proton centre of
mass energy
DIS probes the partonic structure of the proton
Diffraction exchange of color singlet producing
a rapidity GAP in the particle flow
fraction of the p momentum carried by the IP
fraction of the IP momentum carried by the struck
quark
Diff DIS probes the partonic structure of colour
singlet exchange
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Inclusive diffraction ?p ? Xp

• diffractive ?p cross section

• diffractive structure function
• (assumes )

The exchange of color singlet produce a GAP in
the particle flow
? No activity in the forward direction ? Proton
almost intact after the collision
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Selection of events ?p ? Xp with LPS

• Free of p-diss background
• Direct t measurement
• Low acceptance ? low statistics

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Selection of events ?p ? Xp with Mx method

• Properties of Mx distribution
• - exponentially falling for decreasing Mx for
  non-diffractive events
• - flat vs ln Mx2for diffractive events

• High acceptance
• t measurement not possible
• systematics from p-diss (ep?eXN)

Non-diffr.
Diffr.

• Forward Plug Calorimeter (FPC)
• CAL acceptance extended by 1 unit in
  pseudorapidity from ?4 to ?5
• higher Mx and lower W
• if MN gt 2.3 GeV deposits EFPC gt 1 GeV
  recognized and rejected!

D, c, b from a fit to data
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xIP dependence of F2D(3) (LPS method)
97 LPS sample (Higher xIP region) Common xIP
dependence in all bins consistent with the
assumption of Regge factorization
with
by fitting xIP dep. at fixed ß,Q2
xIP dep. of F2D(3) equivalent to W dep. of ds/dMx
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Cross section W dependence (MX method)
98-00 FPC sample (Higher ß region) p-dissociation
events with MNlt2.3 GeV included

• MXlt 2 GeV weak W dep.
• MXgt 2 GeV d?/dMX rises
• rapidly with W

power-like fit
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aIP from diffractive and total ?p scattering
(Mx method)
fit to diffractive cross section data
fit to total cross section

• ?IPdiff higher than soft Pomeron

• Evidence of a rise of ?IPdiff with Q2 ? mild
  Regge factorisation violation

• Same W dep. of diffractive and total cross
  section

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sdiff/ stot W dependence
(Mx method)
Regge th

• low MX strong decrease of
• ?diff/?tot with increasing Q2
• high MX no Q2 dependence !

at W220 GeV
?diff(MXlt35 GeV)/?tot 20 Q2 2.7 GeV ? 10
Q2 27 GeV
In DIS ?diff/?tot independent of W !
Same results from ZEUS-LPS
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F2D(3) /F2 Q2 and xBJ dependences (LPS method)
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The colour dipole picture
Virtual photon fluctuates to qq, qqg states
(colour dipoles)
Provide a good description of the DIS diffractive
data
Two models ?
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Color dipole models
e.g. Saturation Model (Golec-Biernat and M.
Wusthoff)

• pQCD?qq ? r2 ? 1/Q2 (colour transparency)
• As Q2 ? 0, ?qq ? ? violation of unitarity
• Growth by ?qq saturating at ?qq ? ?(?p)

e.g. BEKW model (Bartels, Ellis, Kowalski and
Wüsthoff)
FTqq ß(1- ß), weak Q2 dep. FTqqg (1- ß)? , ln
(1Q2/Q20), Q201GeV2 FLqq only at high ß
energy dep. for both from fits to the data
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Cross section Q2 dependence (LPS)

• Transition to a constant
• cross section as Q2?0
• (similar to total cross section )

• Main features of the data described by BEKW
  parametrization (xIPlt0.01)

(Bartels, Ellis, Kowalski and Wüsthoff)
energy dep. for both (from fits to the data)
FTqq ß(1- ß), weak Q2 dep. FTqqg (1- ß)? , ln
(1Q2/Q20), Q201GeV2 FLqq only at high ß
(medium ß)
(small ß)
qqg fluctuations dominant at low Q2
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F2D(3) Q2 dependence (LPS)
xIP lt 0.01 (diff peak region)
xIP gt 0.01 (exchange of subleading trajectories
important)

• Data well described by BGK (Bartels,
  Golec-Biernat, Kowalski) saturation model
  (xIPlt0.01)

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(Diffractive) hard scattering factorisation
Diffractive DIS, like inclusive DIS, is
factorisable into a hard part and a soft part
QCD Hard Scattering factorizationTrentadue,
Veneziano Berera, Soper Collins
fi/pD(x,Q2,xIP,t) probability to find, with
probe of resolution Q2, in a proton, parton q
with momentum fraction x, under the condition
that proton remains intact and emerges with small
energy loss, xIP, and momentum transfer, t ?
diffractive PDFs are a feature of the proton

Diffractive Deep Inelastic Scattering probes the
diffractive PDFs of the proton relevant when the
vacuum quantum numbers are exchanged
REGGE FACTORISATION
Shape of diffractive PDFs, independent on xIP and
t
Regge motivated IP flux
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NLO QCD fit on LPScharm data

• NLO DGLAP fit, QCDNUM,CTEQ
• MXgt2 GeV, xIPlt0.01, Q2gt2 GeV2
• Regge factorisation assumption
• possible for this small data set
• DL flux
• F2D(3) (xIP,ß,Q2)FIP(xIP) F2IP(ß,Q2)
• initial scale Q22 GeV2
• other PDFs parametrisation tried
• Thorne-Robert variable flavour
• number scheme

• QCD fit describes data
• fractional gluon momentum
• is

Similar to H1!
F2D (3)cc from DESY-03-094

• Positive scaling violation at all values of xIP

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LPS QCD fit compared to Mx data
ZEUS (MX method)
NB fits scaled by 0.69 to account for
p-diss background in Mx data

• Mx data described by the fit in the region of
  overlap LPS-Mx

• Main discrepancies at
• high beta, where
• no LPS data available

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Summary

• Recent data from ZEUS with improved precision
  and extended
• kinematic range
• Data described by dipole models (BEKW,
  saturation)
• Data described by a NLO QCD fit
• Indication that aIP increases with Q2 in
  diffraction
• W dep. of diffractive and total cross section
  similar at high Q2