Diffractive dijets in photoproduction

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Diffractive dijets in photoproduction
single differential cross sections
Shinji Kagawa on behalf of the ZEUS Collaboration
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Diffractive dijets in photoproduction
Parton momenta in the Pomeron ( IP )
reconstructed by jets Dominant process
gluon-intiated process
(e.g. boson gluon fusion) ? Sensitive to
the gluon content of the Pomeron
zIPobs
longitudinal momentum fraction of the IP taken by
the dijet
Direct process total longitudinal momentum of ?
taken by the dijet
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The resolved ? process in diffractive dijets in
photoproduction

• Resolved ? process
• exchanged photon source of partons ? like a
  hadron
• similar to hadron - hadron collision
• dominant process at low x?obs

x?obs
longitudinal momentum fraction of the ? taken by
the dijet
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Factorization breaking in pp collisions
?

• Possible mechanism of suppression secondary
  interactions filling the rapidity gap.
• ? Is there also a suppression of resolved ?
  processes, which are supposedly similar to pp ?

• pp collisions at TEVATRONsuppression of
  diffractive dijetsby a factor of approx.
  10(factorization breaking)

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Suppression of resolved ? processes at NLO

• M.Klasen and G.Kramer made NLO calculations and
  compared them to H1 results.
• No suppression at LO
• At NLO, H1 data is well reproduced when the
  resolved part is suppressed by a factor of 0.34
• Only suppression at NLO ?

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Other kinematic variables
dijet
fraction of electron energy transferred
X
Direct process
longitudinal momentum fraction of the proton
transferred
invariant mass of the system X
X
dijet

• Reconstruction of jets
• longitudinally-invariant kT algorithm

Resolved process
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Kinematic range for measured cross sections

• PHP 0.20 lt y lt 0.85, Q2 lt 1.0 GeV2
• Diffraction xIP lt 0.035
• Dijet ETjet1(2) gt 7.5(6.5) GeV
• ?1.5 lt ?jet1,2 (lab frame) lt 2.0

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Example event
Large rapidity gap (?max) characterizing
diffractive events
e? (not detected)
dijet
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Data sample and selection cuts

• ZEUS 99?00 data (77.6 pb-1)
• Diffractive events selected by requiring (in the
  lab frame)
• E lt 1 GeV from pseudorapidity 4?5
• (in the Forward Plug Calorimeter)
• no cluster with E ? 0.4 GeV from pseudorapidity
  3?4
• (?max cut in the Calorimeter)
• Large data sample 10673 events
• measurements possible up to high jet ET where NLO
• calculations are reliable

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Monte Carlo

• RAPGAP version 3.00/00 (Hannes Jung)
• Structure functions GRV-G-LO ( ? ), H1-fit2 (
  IP )
• Only Pomeron contribution generated (no Reggeon)

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Control plots (I)

• MC reweighted in zIPobs
• ? ? (direct) ? ? (resolved)
• is fitted to data in x?obs
• ? ??? 1.1
• MC has higher ?max than data
• ? largest systematic error

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Control plots (II)

• jet1 highest ET jet
• jets and MX are well described by MC

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Differential cross sections in ET jet1 and ? jet1

• MC area normalized to data by 0.59
• MC describes the shape of jet cross sections well

16 of the cross sections subtracted as p
dissociation background (DESY-03-094)
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Differential cross sections in y and xIP
y electron energy transfer xIP proton
longitudinal momentum transfer ? MC describes the
shapes of data well
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Differential cross sections in MX

• MX invariant mass
• of system X
• Once again, shapes agree

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Differential cross section in zIPobs

• zIPobs sensitive to the parton distribution of
  the diffractive exchange (Pomeron)
• Data is higher at high zIPobs

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Differential cross section in x?obs
resolved enriched
direct enriched

• Data/MC 0.59 (flat)
• No evidence of a resolved suppression with
  respect to direct at LO

?
?
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Summary

• Single differential cross sections of diffractive
  dijets in photoproduction measured using ZEUS
  99?00 data.Adds to the previous ZEUS analysis of
  94 data with very high statistics.
• Results are fairly consistent with RAPGAP
  (v3.00/00 H1-fit2) scaled down by 0.59
• Shape of x?obs well reproduced
• ? No evidence of a suppression of resolved with
  respect to direct at LO