diffractive dijets atdiffractive dijets at hera · diffractive event selection 1) proton...
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Diffractive dijets atDiffractive dijets at HERA
Representing H1 and ZEUS experiments
HERA/LHC workshop May 2008
Alice ValkárováCh l U i it P
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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 b 1 120 b 1• HERA I: 16 pb-1 e-p, 120 pb–1 e+p• HERA II: ∼ 550 pb-1, ∼ 40% polarisation of e+,e-• closed July 2007, still lot of excellent data to analyse……
X
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X
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p
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DIS: Probe structure of proton → F2
Diffractive DIS: Probe structure of
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Diffractive DIS: Probe structure of color singlet exchange → F2
D
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Diffraction and diffraction kinematics
´
Two classes of diffractive events:
Q2~0 → photoproductionQ2>>0 → deep inelastic scattering (DIS)
2HERA: ~10% of low-x DIS events are diffractive
Q >>0 → deep inelastic scattering (DIS)
( )22
22
IP WQMQ
pqppqx X
++≈
⋅′−⋅=
βW momentum fraction of color singlet exchange
22
2
XIP MQQ
xx
+≈=β
W m m m f f g g
XQfraction of exchange momentum, coupling to γ*
2, )( ppt −=t
4-momentum transferd
My
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)( ppt squared
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Diffractive Event Selection1) Proton Spectrometers: ZEUS: LPS (1993-2000) H1: FPS (1995-2007),VFPS (2004-07)t measurementaccess to high xIP rangefree of p-dissociation background at low xIPsmall acceptance low statistics ☠small acceptance low statistics ☠
2) Large Rapidity Gap, H1, ZEUS:Require no activity beyond η max
e p
t not measured, some p-diss background ☠
3) Mx method, ZEUS:Diffractive vs non diffractive: exponential
)ln(tan- η 2θ=
Diffractive vs non-diffractive: exponential fall off vs constant distribution in ln Mx
2
Some p-diss contribution ☠2lnMBdN
diff. non-diff
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ln2ln
xMB
x
CeDMd
dN +∝
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QCD factorizationGet PDF from inclusive diffraction predict cross sections for exclusive diffraction
H d tt i QCDHard scattering QCDmatrix element,perturbativelycalculated, process dependent
Universal diffractive partondensities, identical for all
processesprocesses
),(),,,()( 2*2* QxtxQxfXpp iIP
D
ipartoni
D γσγσ ⋅=→ ∑_ iparton
Dif
*
→ DPDFs – obey DGLAP, universal for diff. ep DIS (inclusive,dijet,charm)
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i*γσ → universal hard scattering cross section (same as in inclusive DIS)
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Proton vertex (Regge) factorizationProton vertex (Regge) factorizationAdditional assumption – there is no proof!
Bte
),/(),(),,,( 2/
2 QxxftxftxQxf IPIP
iIPpIPIPD
i =⋅= β
1)(2/ ),( −= tIP
Bt
IPpIP xetxf α pomeron flux factor pomeron PDF
Exctracted from inclusive diffraction!Fits 2006 A and BFits 2006 A and B
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H1 2006 fits -resultsH1 2006 fits resultsz (zIP)- longitudinal momentum fraction of gluon rel. to
Two fits, A and BFi B l i d
colorless exchange
Fit B: gluon parameterised as aconstant at starting scale
• quarks are very stable• gluons carries ~75% ofgluons carries ~75% ofpomeron momentum
• gluons similar at low z• no sensitivity to gluon
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no sens t v ty to gluonat high z
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Dijets in diffractive DIS, H14 < Q2 < 80 GeV2
0.1 < y < 0.7x 0 03xIIP < 0.03
P*tjet1 > 5.5 GeVP* > 4 GeV
Data 99/00
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, H1j ff ,
For zIP < 0.4 NLO predictionsusing fits 2006 A and B agree with data very wellagree 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 toinclusive cross section to gluonPDF especially at large zIP →DF sp c a y at arg zIPuse jets to combined fits!
JHEP 0710:042,2007
Largestdifference Largest
differenceAA
B Ajets
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Dijets in diffractive DIS, ZEUS
Eur.Phys.J.C52: 83 (2007)
Data 99/00E*tjet1 > 5 GeV
R=data/NLO(ZEUS LPS)
The best agreement ofThe best agreement of data and NLO for H1 2006 fit B andMRW 2006
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11H1, ZEUS: for DIS dijets factorization holds…..
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Hadron-hadron collisions
γ*p
Exporting DPDFs fromHERA to Tevatron…..
pp
γ pxIP integrated effective DPDFsfrom CDF single diff. dijets (run I)
Effect is known since 10 years!
Factorization broken by
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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
jetszOBS
pEpE
xx)()( −
== ∑γγ
hadronszpE )( −
direct photoproduction (Q2≃0):photon directly involved in hard scattering
1
resolved photoproduction (Q2≃0): photon fluctuates into hadronic
system, which takes part in hadronicscattering,dominant at Q2≃0
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xγ=1(at parton level)
g, Qxγ<1(at parton level)
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Photoproduction as hadronic process
HERA resolved photoproduction
Secondary interactionsbetween spectatorsp
Typical models that describe suppression at Tevatron assume secondaryinteractions of spectators as the cause:
resolved contribution expected to be suppressed by factor 0 34
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resolved contribution expected to be suppressed by factor 0.34 (Kaidalov,Khoze,Martin,Ryskin:Phys.Lett.B567 (2003),61)
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Photoproduction, γ⋆p, Q2 ⋍ 0
xγ - fraction of photon’s momentum in hard subprocessExpectation: Gap Survival Expectation: Gap Survival
P b bili 1
jetszOBS
pEpE
xx)()( −
== ∑γγ
p pProbability =1 Probability < 1
hadronszpE )( −
direct photoproduction (Q2≃0):photon directly involved in hard scattering
1
resolved photoproduction (Q2≃0): photon fluctuates into hadronic
system, which takes part in hadronicscattering,dominant at Q2≃0
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xγ=1(at parton level)
g, Qxγ<1(at parton level)
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One year ago….
H1 E 5 G V i f f t 0 5H1: Etjet1 > 5 GeV suppression of factor ~0.5ZEUS: 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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Different phase space of both analyses ……..?
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Et dependence of suppression?Et dependence of suppression?H1 ZEUS
From the DIS 2008 talk of W SlomińskiFrom the DIS 2008 talk of W.Slomiński,ZEUS results
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Double ratios of γ⋆p & DIS(d t /NLO)(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 dependencebut
what about Etj t1 dependence??what about Etjet1 dependence??
Etjet1>5 GeV
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H1 – double ratio, Et dependenceH1 double ratio, Et dependence Double ratio of Data/NLO for h d i d DISphotoproduction and DIS
→
Figure extracted from published results
Double ratio is within errors Et dependent!
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New H1 analysis – data 99/00New H1 analysis data 99/00Tagged photoproduction, luminosity 3x larger than for 97diffractive events found by Large Rapidity Gap method (LRG)diffractive events found by Large Rapidity Gap method (LRG)
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Two cut scenariosTwo cut scenariosTo crosscheck previous H1 results To approach closest to ZEUS cuts
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Theoretical predictionsTheoretical predictions
2 programs for NLO calculations, 3 sets of DPDFs:2 programs for NLO calculations, 3 sets of DPDFs
Frixione/Ridolfi → H1 2006 Fit Ainclusive measurement
H1 2006 Fit BH1 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 crosscheckNLO programs
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Lower Et cut scenario
Integrated survival probabilities (ISP)
Within errors no difference in ISPusing different DPDFs
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No difference in survival probabilitiesfor 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 datah NLOthan NLO
Hadronization corrections
δ =MC(hadr)/MC(parton)δhadr=MC(hadr)/MC(parton)
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Higher Et cut scenario
Now much more direct-like“ events thanNow much more „direct like events thanin low Et analysis, peak at higher xγ
Integrated survival probabilities (ISP)Integrated survival probabilities (ISP)
Larger ISP than for lower Et cut scenario → more close to ZEUS results!!!
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more close to ZEUS results!!!
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Higher Et cut scenario
Et dependence not excluded but cannotbe independently verified
ZEUS – W.Slomiński
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What DPDF is the „best“?
The most sensitivevariable → zIP
Fits 2006 A,B valid onlyto zIP =0.8Fit 2007 jets to z P = 0 9Fit 2007 jets to zIP = 0.9
Poorly constraint DPDFsgive large uncertainties atgive large uncertainties atlarge zIP.Fits 2006 A and 2007 Jetsrepresent extremes.p
Fit 2006 B is in the middle….
L E t i
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Lower Et cut scenario Higher Et cut scenario
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Summary1 d h d d ld 1 l f d• new H1 dijet photoproduction data – older H1 results confirmed
– within errors is gap survival probability (GSP) ~ 0.5.
i hi h E t s i (si il t ZEUS) GSP is 0 6• in higher Et cut scenario (similar to ZEUS) GSP is ~ 0.6, more close to ZEUS results.
• hint that GSP is dependent on E of the leading jet• 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 andthe evidence that GSP is not different for direct andresolved events remains (originally not expected )
• the problem is rather the theoretical uncertainties than pthe lack of experimental results! NNLO????
• important for LHC predictions….(depend on DPDFs from HERA,d G P)
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and GSP)