Transcript Diffractive dijets at HERA

Diffractive dijets at
HERA
Representing H1 and ZEUS experiments
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: ∼ 550 pb-1, ∼ 40% polarisation of e+,e-
DIS: Probe structure of proton → F2
e
Diffractive DIS: Probe structure of
color singlet exchange → F2D
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Diffraction and diffraction kinematics
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 single exchange
My
t
x
Q2

 2
x IP Q  M X 2
fraction of exchange momentum, coupling to γ*
t  ( p  p, )2
4-momentum transfer
squared
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Diffractive Event Selection
1) Proton Spectrometers:
ZEUS: LPS (1993-2000)
H1: FPS (1995-2008),VFPS (2004-08)
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
η  - ln(tan θ2 )
3) Mx method, ZEUS:
Diffractive vs non-diffractive: exponential
fall off vs constant distribution in ln Mx2
Some p-diss contribution ☠
diff. non-diff
dN
B ln M x2
 D  Ce
2
d ln M 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
Universal diffractive parton
densities, identical for all
processes
 D ( * p  Xp) 
2
 *i
2
f
(
x
,
Q
,
x
,
t
)


(
x
,
Q
)
i
IP
D
parton_ i
D
f i → DPDFs – obey DGLAP, universal for diff. ep DIS (inclusive,dijet,charm)
  *i → universal hard scattering cross section (same as in inclusive DIS)
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Proton vertex (Regge) factorization
Additional assumption – there is no proof!
fi ( x, Q 2 , xIP , t )  f IP / p ( xIP , t )  fi (   x / xIP , Q 2 )
D
f IP / p ( x IP , t ) 
IP
e Bt
2 ( t ) 1
x IP
pomeron flux factor
pomeron PDF
Exctracted from inclusive diffraction!
Fits 2006 A and B
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H1 2006 fits -results
z (zIP)- longitudinal momentum
fraction of gluon rel. to
colorless exchange
Two fits, A and B
Fit B: gluon parameterised as a
constant at starting scale
• quarks are very stable
• gluons carries ~75% of
pomeron momentum
• gluons similar at low z
• no sensitivity to gluon
at high z
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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
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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…..
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H1 QCD jet fit
Low sensitivity of fits to
inclusive cross section to gluon
PDF especially at large zIP →
use jets to combined fits!
JHEP 0710:042,2007
Largest
difference
Largest
difference
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Dijets in diffractive DIS, ZEUS
Eur.Phys.J.C52: 83 (2007)
E*tjet1 > 5
GeV
Data 99/00
R=data/NLO(ZEUS LPS)
The best agreement of
data and NLO
for H1 2006 fit B and
MRW 2006
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H1, ZEUS: for DIS dijets factorization holds…..
Hadron-hadron collisions
γ*p
Exporting DPDFs from
HERA to Tevatron…..

xIP integrated effective DPDFs
from CDF single diff. dijets (run I)
pp
If for HERA incl. data
rapidity gap survival factor =1,
at Tevatron ~0.1
Factorization broken by
β-dependent factor ~ 10 !
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Photoproduction, γ⋆p, Q2→0
xγ - fraction of photon’s momentum
in hard subprocess
x  x
OBS
(Q2≃0):
direct photoproduction
photon directly involved in hard
scattering
xγ=1
Expect: gap survival probablity= 1
(E  p )


z
jets
( E  pz ) hadrons
resolved photoproduction (Q2≃0):
photon fluctuates into hadronic system,
which takes part in hadronic
scattering,dominant at Q2≃0
xγ<1
Expect: gap survival probability < 1 13
Theory (Kaidalov at al) ~0.34
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 ……..?
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Et dependence of suppression?
H1
ZEUS
From the DIS 2008 talk of W.Slomiński,
ZEUS results
H1 and ZEUS observe the data have harder Et slope than NLO
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Double ratios of γ⋆p & DIS
(data/NLO)
H1 collab. Eur.Phys.J,C51 (2007),549
Very useful – full or partial cancellation of many uncertainties
(energy scales for data, DPDFs used…etc ).
There is no clear W dependence
but
what about Etjet1 dependence??
Etjet1>5 GeV
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H1 - ratio photoproduction/DIS
Double ratio of Data/NLO for
photoproduction and DIS
→ uncertainties of diffractive PDFs
cancel!
Figure derived from published results
Double ratio is within errors Et dependent!
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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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Theoretical predictions
2 programs for NLO calculations, 3 sets of DPDFs:
Frixione/Ridolfi → H1 2006 Fit A
H1 2006 Fit B
H1 2007 Fit Jets →
inclusive measurement
DIS dijets
Kramer/Klasen → H1 2006 Fit B
The aim was to estimate the effect on DPDF used and to crosscheck
NLO programs
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Lower Et cut scenario
Integrated survival probabilities (ISP)
Within errors no difference in ISP
using different DPDFs
No difference in survival probabilities
for resolved and direct regions of xγ,
like in previous H1 and ZEUS analyses
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Lower Et cut scenario
Another hint of Et harder slope for data
than NLO
Hadronization corrections
δhadr=MC(hadr)/MC(parton)
ZEUS – W.Slominski
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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!!!
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Higher Et cut scenario
Et dependence not excluded but cannot
be independently verified
ZEUS – W.Slomiński
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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
Fits 2006 A and 2007 Jets
represent extremes.
Fit 2006 B is in the middle….
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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 )
• to understand better diffractive dijets in photoproduction is
challenging!!!
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