Transcript Diffractive dijets at HERA

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 →√s=318
GeV
• two experiments: H1 and ZEUS
• HERA I: 16 pb-1 e-p, 120 pb–1 e+p
• 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
8.7.2008
Low x workshop, Kolimpari
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Diffraction and diffraction kinematics
Two classes of diffractive events:
Q2~0 → photoproduction
Q2>>0 → deep inelastic scattering ´(DIS)
HERA: ~10% of low-x DIS events are diffractive
q   p  p  Q 2  M X
xIP 
 2
q p
Q W 2
2
W momentum fraction of color singlet exchange
My
t
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x
Q2

 2
x IP Q  M X 2
fraction of exchange momentum, coupling to γ*
t  ( p  p, )2
Low x workshop, Kolimpari
4-momentum transfer
squared
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Diffractive Event Selection
1) Proton Spectrometers:
ZEUS: LPS (1993-2000)
H1: FPS (1995-2007),VFPS (2004-07)
t measurement
access to high xIP range
free of p-dissociation background at low xIP
small acceptance  low statistics ☠
2) Large Rapidity Gap, H1, ZEUS:
Require no activity beyond η max
t not measured, some p-diss background ☠
p
e
3) Mx method, ZEUS:
Diffractive vs non-diffractive: exponential
fall off vs constant distribution in ln Mx2
Some p-diss contribution ☠
diff. non-diff
η  - ln(tan θ2 )
dN
B ln M x2
 D  Ce
2
d ln M x
8.7.2008
Low x workshop, Kolimpari
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Dijets in diffractive ep scattering
Resolved photon
LO
Pointlike photon
xγ - fraction of photon’s momentum
in hard subprocess
x  x
OBS
(E  p )


z
jets
( E  pz )hadrons
DIS, direct PHP
photon directly involved in hard
scattering
xγ=1
(at parton level),
due to hadronization and resolution
not exactly true for measured xγ
8.7.2008
resolved PHP
photon fluctuates into hadronic system,
which takes part in hadronic
scattering,dominant at Q2≃0
xγ<1
(at parton level)
Low x workshop, Kolimpari
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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
Universal diffractive parton
densities, identical for all
processes
 D ( * p  Xp ) 

f i ( x, Q 2 , x IP , t )    *i ( x, Q 2 )
D
parton _ i
D
fi → DPDFs – obey DGLAP, universal for diff. ep DIS (inclusive,dijet,charm)
  *i
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→ universal hard scattering cross section (same as in inclusive DIS)
Low x workshop, Kolimpari
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Dijets in diffractive DIS, H1
4 < Q2 < 80 GeV2
0.1 < y < 0.7
xIIP < 0.03
Data 99/00
P*tjet1 > 5.5 GeV
P*tjet2 > 4 GeV
-3. < η*jets < 0.
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,0710:042,(2007)
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Low x workshop, Kolimpari
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Dijets in diffractive DIS, H1
For zIP < 0.4 NLO predictions using fits 2006 A and B
agree with data very well
Combined QCD fit for inclusive and dijet DIS data…..
A
Largest
difference
B
jets
8.7.2008
Low x workshop, Kolimpari
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Dijets in diffractive DIS, ZEUS
Eur.Phys.J.C52: 83 (2007)
E*tjet1 > 5 GeV
5 < Q2 < 100 GeV2
Data 99/00
R=data/NLO(ZEUS LPS)
Conclusions:
the best agreement of
data and NLO
for H1 2006 fit B and
MRW 2006
8.7.2008
H1, ZEUS: for Low
DIS
dijets factorization holds…..
x workshop, Kolimpari
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Photoproduction as hadronic process
Factorization broken by β dependent factor ~ 10
HERA resolved photoproduction
Secondary interactions
between spectators
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,Ryskin:Phys.Lett.B567 (2003),61)
8.7.2008
Low x workshop, Kolimpari
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Dijets in photoproduction, one year ago….
H1:
Etjet1 > 5 GeV suppression of factor ~0.5
ZEUS: Etjet1 > 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 ……..?
8.7.2008
Low x workshop, Kolimpari
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Et dependence of suppression?
From the DIS 2008 talk of W.Slomiński,
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
8.7.2008
Low x workshop, Kolimpari
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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)
8.7.2008
Low x workshop, Kolimpari
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Two cut scenarios
To crosscheck previous H1 results
8.7.2008
To approach closest to ZEUS cuts
Low x workshop, Kolimpari
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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
δhadr=MC(hadr)/MC(parton)
No difference in survival probabilities
for resolved and direct regions of xγ,
like in previous H1 and ZEUS analyses
8.7.2008
Low x workshop, Kolimpari
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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
8.7.2008
Low x workshop, Kolimpari
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Higher Et cut scenario
Now much more „direct-like“ events than
in low Et analysis, peak at higher xγ
Integrated survival probabilities (ISP)
Larger ISP than for lower Et cut scenario →
more close to ZEUS results!!!
8.7.2008
Low x workshop, Kolimpari
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Higher Et cut scenario
Et dependence not excluded but cannot
be independently verified
ZEUS – W.Slomiński
8.7.2008
Low x workshop, Kolimpari
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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 R=0.31 (0.29) for H1 and R=0.53 (0.45) for ZEUS published analyses
H1, 2007
8.7.2008
Low x workshop, Kolimpari
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
8.7.2008
Low x workshop, Kolimpari
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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 data&NLO
for xγ and also for
other distributions!!!
8.7.2008
Low x workshop, Kolimpari
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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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Low x workshop, Kolimpari
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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
8.7.2008
Higher Et cut scenario
Low x workshop, Kolimpari
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