Marcella Capua

1
Results on inclusive diffraction from ZEUS

• Marcella Capua
• Calabria University

• Diffractive kinematics
• Experimental method
• F2D at high Q2 (EPS 01)
• vs W,Q2 (DESY 02-029)
• vs Q2 (NEW!)

DIS 2002 Cracow, Poland April 29 - May 5, 2002
2
Kinematics of inclusive diffraction
Diffractive ?p interactions at HERA exploiting
the hadronic properties of the photon
Q2 - q2 x Q2/2pq W2 (q p)2 MX 2
(qP-P)2
MY
NB if proton not detected, background from
proton dissociative events
3
Measurement of inclusive diffraction

• ?p cross section

• Energy dependence in Regge theory

4
Selection of inclusive diffractive events
MX method

• Exploit properties of MX distribution
• Flat vs lnMX2 for diffractive events
• Exponentially falling for decreasing MX for
  non-diffractive events

• Good statistics!
• t measurement not possible
• Large systematics from p-diss

5
Selection of inclusive diffractive events
Leading Proton Spectrometer (LPS) method
Diffractive peak

• Measurement of t distribution
• Free of p-diss background
• Lower acceptance

xL
6
Data samples
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Factorization

• Regge factorization - resolved IP model (IP
  with partonic structure)

(Breit frame)
Regge motivated pomeron flux Structure
functions evolve according to DGLAP
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F2D(4) (b,Q2,xIP,t)
Common xIP dependence in all bins consistent with
the assumption of factorization of the structure
function
from Regge factorization
Extract aIP(0) by fitting xIP dep. at fixed ß,Q2
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F2D(3) (b,Q2,xIP)
Use measured t-slope to extrapolate in t
F2D(3) results from H1(FPS method) and ZEUS (LPS
method) similar
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Data samples
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W dep. of the diffractive cross section
Fit to the data a constant power of W
Ai free to vary on Q2, MX bins
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How universal is the IP?
aIP(0) measured in diffraction at different Q2
Within the uncertainties there is no evidence of
a change of aIP(0) with Q2
Within the uncertainties there is no evidence of
a change of aIP(0) with Q2
no evidence of a change of aIP(0) with Q2
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Energy dep. of the diffractive cross section
Regge theory
No statistically significant change from the low
Q2 to the DIS regime
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Energy dep. of diffractive to inclusive ratio
Regge theory
Fit
at high Q2 not Regge-like
at low Q2 Regge-like
Regge prediction is not fulfilled in the DIS
regime
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Color dipole models


Color dipole scatters elastically off the proton
(LO, p rest frame)
IP described as 2-gluon color singlet exchange
Provide a good description of the DIS diffractive
data
e.g. Saturation Model (Golec-Biernat and M.
Wusthoff) (cf. DIS00, DIS01)
e.g. BEKW model (Bartels et al.,Eur. Phys. J.
C7,443 (1999))
FTqq ß(1- ß), no 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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Q2 dep. of the diffractive cross section
Going from the DIS regime to the low Q2 we
observe a change in the Q2 dependence of the
diffractive cross section This is similar to what
observed for the total cross section s?p at Q2
1 GeV2
Main features of the data described by BEKW
parametrization
? fluctuations dominant - for large
MX - for low Q2
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Q2 dep. of diffractive to inclusive ratio
At low Q2 the ratio shows little dependence on Q2
At high Q2 the diff cross section decreases more
rapidly than stot
Main features of the data described by BEKW
parametrization
? fluctuations dominant - for large
MX - for low Q2
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Data samples
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Q2 dep. of the diffractive cross section
New preliminary 97 LPS data! Higher W, lower Q2
and more data at high MX
Same behaviour in the new region
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Summary

• New ZEUS F2D and dsDiffdMX results covering a
  wide kinematical range

• W dependence of diffractive and total cross
  section becomes similar at large Q2 contrary to
  what happens at low Q2

• Q2 dependence of the diffractive softens
  considerably for Q2 ? 0

• Features of the data consistent with models
  assuming and fluctuations of the photon