Jets in diffraction and factorization at HERA

1
Jets in diffraction and factorization at HERA
On behalf of H1 and ZEUS collaborations

• Alice Valkárová
• Charles University, Prague

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HERA collider experiments

• 27.5 GeV electrons/positrons on 920 GeV protons
  ?vs318 GeV
• two experiments H1 and ZEUS
• HERA I 16 pb-1 e-p, 120 pb1 ep
• HERA II 500 pb-1, 40 polarisation of e,e-
• closed July 2007, still lot of excellent data to
  analyse

DIS Probe structure of proton ? F2
e
Diffractive DIS Probe structure of color
singlet exchange ? F2D
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Diffraction and diffraction kinematics
Two classes of diffractive events Q20 ?
photoproduction Q2gtgt0 ? deep inelastic scattering
(DIS)

HERA 10 of low-x DIS events are diffractive
momentum fraction of color singlet exchange
My
fraction of exchange momentum, coupling to ?
t
4-momentum transfer squared
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Diffractive Event Selection
p
e
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Dijets in diffractive ep scattering
Resolved photon
LO
Pointlike photon
x? - fraction of photons momentum in
hard subprocess
DIS, direct PHP
resolved PHP

• photon fluctuates into hadronic system, which
  takes part in hadronic scattering,dominant at
  Q2?0
• x?lt1
• (at parton level)

photon directly involved in hard scattering
x?1 (at parton level), due to hadronization and
resolution not exactly true for measured x?
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QCD factorization
Get PDF from inclusive diffraction
predict cross sections for exclusive diffraction
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
? DPDFs obey DGLAP, universal for diff. ep DIS
(inclusive,dijet,charm)
? universal hard scattering cross section (same
as in inclusive DIS)
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Dijets in diffractive DIS, H1

• lt Q2 lt 80 GeV2
• 0.1 lt y lt 0.7
• xIIP lt 0.03
• Ptjet1 gt 5.5 GeV
• Ptjet2 gt 4 GeV
• -3. lt ?jets lt 0.

Data 99/00
zIP is the most sensitive variable to test
gluonic part of DPDFs difference between fits A
and B at high zIP. Data agree with NLO
prediction, NLO with fit B is more close to data
published, JHEP,0710042,(2007)
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Dijets in diffractive DIS, H1
For zIP lt 0.4 NLO predictions using fits 2006 A
and B agree with data very well Combined QCD
fit for inclusive and dijet DIS data..
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Dijets in diffractive DIS, ZEUS
Eur.Phys.J.C52 83 (2007)
Etjet1 gt 5 GeV 5 lt Q2 lt 100 GeV2
Data 99/00
Rdata/NLO(ZEUS LPS)
Conclusions the best agreement of data and NLO
for H1 2006 fit B and MRW 2006
H1, ZEUS for DIS dijets factorization holds..
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Photoproduction as hadronic process
Factorization broken by ß dependent factor 10
HERA resolved photoproduction
Secondary interactions between spectators
Jets in photoproduction thought to be ideal
testing ground for rescattering
Rescattering leads to factorization breaking and
rapidity gap fill up suppression of cross
section 1 (rapidity survival probability)
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, one year ago.
H1 Etjet1 gt 5 GeV suppression of factor
0.5 ZEUS Etjet1 gt 7.5 GeV weak (if any)
suppression (0.6-0.9) Neither collaboration sees
difference between the resolved and direct
regions, in contrast to theory! Possible
explanation of differences between H1 and ZEUS
(DIS 2007) Different phase space of both analyses
..?
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Et dependence of suppression?
From the DIS 2008 talk of W.Slominski, ZEUS
results
Double ratio NLO/data for photoproduction and
DIS Cancelations of several uncertainties
(DPDFs for example) Figure extracted from the
published results of H1
H1 and ZEUS observe the data have harder Et slope
than NLO
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New H1 analysis data 99/00

• Tagged photoproduction, luminosity 3x larger
  than for 97
• diffractive events found by Large Rapidity Gap
  method (LRG)

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Two cut scenarios
To crosscheck previous H1 results To
approach closest to ZEUS cuts
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Lower Et cut scenario
2 programs for NLO calculations, 3 sets of DPDFs
Frixione/Ridolfi ? H1 2006 Fit A
H1 2006 Fit B
H1 2006 Fit Jets Kramer/Klasen ? H1
2006 Fit B (thanks to Michael K.)
Hadronization corrections dhadrMC(hadr)/MC(parto
n)
No difference in survival probabilities for
resolved and direct regions of x?, like in
previous H1 and ZEUS analyses
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Lower Et cut scenario
Integrated survival probabilities (ISP)
Within errors no difference in ISP using
different DPDFs
Another hint of Et harder slope for data than NLO
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Higher Et cut scenario
Now much more direct-like events than in low Et
analysis, peak at higher x?
Integrated survival probabilities (ISP)
Larger ISP than for lower Et cut scenario ? more
close to ZEUS results!!!
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Higher Et cut scenario
Et dependence not excluded but cannot be
independently verified
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Do we need global suppression?
M.Klasen,G.Kramer Review of factorization
breaking in diffractive photoproduction of
dijets, DESY 08-074, Modern Phys.Lett.A,2008, an
attempt to interprete ZEUS and H1 results with
NLO resolved only suppression R0.31 (0.29) for
H1 and R0.53 (0.45) for ZEUS published analyses
H1, 2007
ZEUS, 2007
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Global and resolved only suppression
H1 lower Et cut scenario
Global suppression 0.53
Data H1 preliminary, suppression of NLO resolved
component 0.3 (see KK paper)
Much worse agreement than for global suppression
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Global and resolved only suppression
H1 lower Et cut scenario
Global suppression 0.53
Data H1 preliminary, suppression of NLO resolved
component 0.3, (see KK paper)
No Et dependence but prize is worse agreement
dataNLO for x? and also for other
distributions!!!
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Summary

• new H1 dijet photoproduction data older H1
  results confirmed
• within errors is gap survival probability
  (GSP) 0.5.
• in higher Et cut scenario (similar to ZEUS) GSP
  is 0.6,
• more close to ZEUS results.
• hint that GSP is dependent on Et of the leading
  jet,
• for low Et jets seems to be suppression more
  significant.
• the evidence that GSP is not different for
  direct and
• resolved events remains (originally not
  expected )
• the problem are rather the theoretical
  uncertainties than
• the lack of experimental results! (see huge
  scale uncertainties).
• important for LHC predictions.(depend on DPDFs
  from HERA,
• and GSP)

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What DPDF is the best?
The most sensitive variable ? zIP Fits 2006 A,B
valid only to zIP 0.8 Fit 2007 jets to zIP
0.9 Poorly constraint DPDFs give large
uncertainties at large zIP. Fits 2006 A and 2007
Jets represent extremes. Fit 2006 B is in the
middle.
Lower Et cut scenario
Higher Et cut scenario