29.6.2005 Low x meeting, Sinai 2005 1
Alice Valkárováon behalf of H1 collaborationLOW x meeting 2005, Sinaia
H1 measurements of the structureof diffraction and tests of factorisation
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HERA: ~10% of low-x DIS events are diffractive
study QCD structure of high energy diffraction with virtual photon
e´
e
´
´
DIS: Probe structure of proton → F2
Diffractive DIS: Probe structure of color singlet exchange → F2D
HERA experiments and diffraction
29.6.2005 Low x meeting, Sinai 2005 3
Momentum fraction of proton carried by color singlet exchange:
Momentum fraction of color singlet carried by struck quark:
Large rapidity gap between leading proton p´ and X
22
22
IP WQ
MQ
pq
ppqx X
22
2
XIP MQ
Q
x
x
´
W
ηmax ´
Diffraction kinematics
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QCD factorisation
inclusive dijethard scattering QCD matrix element, perturbativelycalculated, process dependent
Universal diffractive parton densitiesidentical for all processes
get PDFs from inclusive diffraction ⇨ predict cross sections for exclusive diffraction
),(),,,()( 2*2
_
* QxtxQxfXpp iIP
D
ipartoni
D i*
Difuniversal hard scattering cross section (same as in inclusive DIS)diffractive parton distribution functions → obey DGLAPuniversal for diffractive ep DIS (inclusive, di-jets, charm)
• proven for DIS (J.Collins (1
998))
• not proven for photoproduction!
29.6.2005 Low x meeting, Sinai 2005 5
σdiff = flux(xP) · object (β,Q2)
Results from inclusive diffractionβ Q2
Reduced cross section frominclusive diffractive data
• get diffractive PDFs from a NLO (LO) DGLAP QCD Fit to inclusive data from 6.5 GeV2 to 120 GeV2
• extrapolation of the Fit to lower Q2
to higher Q2
gives a reasonably good description of inclusive data from 3.5 GeV∼ 2 –1600 GeV2
Regge factorisation is an additional assumption, there is no PROOF!!
),/(),(),,,( 2/
2 QxxftxftxQxf IPIPiIPpIPIP
Di
pomeron flux factor pomeron PDF
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Diffractive Parton Densities
• determined from NLO QCD analysis of diffractive structure function• more sensitive to quarks• gluons from scaling violation, poorer constraint• gluon carries about 75% of pome- ron momentum• large uncertainty at large zP
Assuming factorisation holds,the jet and HQ cross sections givebetter constraint on the gluon density
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Jet and HQ production
Hard scale is ET of the jet or HQ massDirect access to gluon densityCan reconstruct zP in dijet events
• tests of universality of PDF’s (=QCD factorisation)
• test of DGLAP evolution
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Charm cross section (DIS)
NLO calculations HVQDIS(Harris & Smith)
Good agreement within experimental & theoreticaluncertainties.
Good description of diffractive D* production in DIS (2GeV2 <Q2<100 GeV2)
NLO calculations with PDFs from inclusive diffraction
Factoris
ation holds !
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Dijets in DIS
NLO calculations = diffractive extension of DISENT Catani&Seymour (Nucl.Phys.B485 (1997) 29),
interfaced to diffr.PDFs of H1Hadronisation corrections – RAPGAP MC
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Dijets in DIS
• NLO corrections to LO are significant – factor 1.9• excess at high xγ is kinematically connected with the lack of events with ηlab of jets < -0.4 in comparison with NLO
Good agreement withNLO within exp.&theor.uncertainties
Fact
oris
atio
n hol
ds!
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pp
γ*p
CDF Tevatron data:At Tevatron HERA PDF’sdo not work….????
Dijet cross section factor 5-10 lowerthan the QCD calculation using HERAPDFs
?
Break
down o
f fac
toris
atio
n!
Exporting PDFs from HERA tothe Tevatron.........
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Direct and resolved processes at HERA
xγ - fraction of photon’s momentum in hard subprocess
DIS (Q2>5GeV2) and direct photoproduction (Q2 0):≃• photon directly involved in hard scattering
• xγ=1
Resolved photoproduction:• photon fluctuates into hadronic system, which takes part in hadronic scattering• dominant at Q2 0≃
• xγ<1
unsuppressed!
suppressed!
hadronsz
jetszOBS
pE
pExx
)(
)(
?
?
29.6.2005 Low x meeting, Sinai 2005 13
Photoproduction as hadronic process
resolved contribution expected to be suppressed by factor 0.34 (Kaidalov,Khoze,Martin,Ryskin:Phys.Lett.B567 (2003),61)
Typical models that describe suppression at Tevatron assume secondaryinteractions of spectators as the cause:
HERA resolved photoproduction
Secondary interactionsbetween spectators
Jets in photoproductio
n thought to
be
ideal testin
g ground for re
scatterin
g
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Dijets in photoproduction
The same kinematical region as for DIS
• NLO overestimates the cross section by factor 2∼ • both direct and resolved are suppressed• RAPGAP LO – good description
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Dijets in photoproduction
If only resolved part is suppressed (by factor 0.34, according to Kaidalov et al.)⇨ data are not described by NLO
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Ratio:data over NLO prediction
• no suppression observed for DIS• overall suppression factor of about 2 observed for both resolved and direct components in photoproduction• suppression is independent of the cms energy W
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Summary
Dijets in DIS & D* cross section: • agree with the NLO prediction with the H1 2002 diffractive
pDFs
• factorisation holds (assuming PDF is correct)
Dijets in photoproduction:
• to investigate the puzzle of disagreement of HERA/Tevatron data (expectation: resolved will be suppressed and direct not)
• data are half of NLO prediction – both resolved and direct are suppressed ⇨ conflict with the theoretical expectation
More ideas?
18
LPS proton vs Rapidity Gap
Large rapidity gap selection:
MY<1.6 GeV and |t|<1 GeV2
LPS proton selection: MY= mp
extrapolated to |t|<1 GeV2
Good agreement between two methods and two experiments
Data well described by
H1 QCD fit to LRG data
ZEUS Mx data should be scaled by 0.69 to account for p-diss
M.Kapishin, Inclusive diffraction at HERA 18
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H1 and ZEUS