Resent results on inclusive diffraction

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Resent results on inclusive diffraction

• Marcella Capua
• Calabria University and INFN

• New set of results on inclusive
  diffraction obtained by H1 and ZEUS from very
  low to high Q2
• New NLO DGLAP QCD fit from H1
• Comparison with F2 results
• Comparison with models

on behalf of
Diffraction 2002 Alushta, Crimea 29th Aug 5th
Sept 2002
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Kinematics of inclusive diffraction
Diffractive ?p interactions at HERA exploiting
the hadronic properties of the photon
4-momentum trasfer at the p vertex
fraction of p momentum
fraction of the p momentum carried by the IP
fraction of the IP momentum carried by the struck
parton
NB if proton not detected, background from
proton dissociative events
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How the data are presented

• There are three different methods used by H1 and
  ZEUS to select diffraction
• Rapidity gap (H1)
• Mass method (ZEUS)
• Leading proton spectrometer (H1 and ZEUS)
• Results presented in different forms
• H1 Reduced cross section srD(3) (b, xIP, Q2)
  that take in account the FL contribution
• ZEUS Assume FL0 in the kinematic range used and
  two different sets of variables (b, xIP, Q2) in
  DIS and (W, MX, Q2) at low Q2

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Measurement of inclusive diffraction at ZEUS

• ?p cross section

• Energy dependence in Regge theory

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Measurement of inclusive diffraction at H1

• Diffractive reduced cross section srD

• 3-fold differential diffr. structure function
  (t integrated)

• Energy dependence in Regge theory

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Selection of inclusive diffraction at ZEUS
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

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

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

xL
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Selection of inclusive diffraction at H1

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

• Require Large Rapidity Gap
• Kinematics measured from X system
• Integrate over t and MY
• Higher acceptance

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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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ds/dt measurement
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Energy dependence of the cross section

• no Q2 dependences
• the xIP dep. not in agreement between ZEUS and H1
  at low xIP
• Regge theory expect shrinkage

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F2D(3) (b,Q2,xIP) at high Q2
Common xIP dependence in all bins consistent with
the assumption of factorization of the structure
function
from Regge factorization
by fitting xIP dep. at fixed ß,Q2
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F2D(3)(b,Q2,xIP) and ds/dMX(Q2,W,MX) at low Q2
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W dep. of the diffractive cross section at very
low Q2
Fit to the data a constant power of W
Ai free to vary on Q2, MX bins
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srD(3) (b,Q2,xIP) from low to high Q2
? 1.5ltQ2lt12 GeV2 ? 6.5ltQ2lt120 GeV2 ? 2.5ltQ2lt20
GeV2 Agreement between methods High precision
measurement of b and Q2 dependences Red points
used for DGLAP QCD interpretation and aIP(0)
calculation
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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
The line aIP(0)l1 is obtained from fits to the
inclusive small x proton structure function data
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How universal is the IP?
Dominant error due to the FL understimation ?
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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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Energy dep. of diffractive to inclusive ratio
MXlt2GeV the ratio is falling with W not so
evident for high MX values Strong decreasing of
the ratio with the increasing of Q2 not so
evident going on in MX or going down in b
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Energy dep. of diffractive to inclusive ratio
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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 the diffractive cross section
Reasonable description of the data at high and
medium b (qq dominate) qqg fluctuation
insufficient at low b and high Q2
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Q2 dep. of the diffractive cross section
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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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NLO DGLAP QCD fit
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b dependences
rising for b ?1 for low Q2
There is no evidence of the b and Q2 dependence
when xIP changes ? Regge factorisation
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Q2 dependences
positive scaling violations expect for largest b
(gluon dominance)
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Gluon fraction and FLD
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Factorisation tests with hadronic final state
observables
Use dPDFs to predict diffractive final state
cross sections Prediction one order of magnitude
below CDF data confirm a breakdown of
factorization in diffraction between pp and ep
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Summary

• New ZEUS and H1 F2D and cross section 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
• b parameters without Q2 dep and with xIP dip

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

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