Transcript H1 measurements of the structure of diffraction and tests

H1 measurements of the structure
of diffraction and tests of
factorisation
Alice Valkárová
on behalf of H1 collaboration
LOW x meeting 2005, Sinaia
29.6.2005
Low x meeting, Sinai 2005
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HERA experiments and diffraction
´
HERA: ~10% of low-x DIS events are diffractive
 study QCD structure of high energy diffraction
with virtual photon
DIS: Probe structure of proton → F2
e´
e
´
Diffractive DIS: Probe structure of color singlet exchange → F2D
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Diffraction kinematics
Large rapidity gap between leading proton p´ and X
ηmax
´
W
´
Momentum fraction of proton carried by
color singlet exchange:
Momentum fraction of color singlet carried
by struck quark:
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q   p  p Q 2  M X
xIP 
 2
q p
Q W 2
2
x
Q2

 2
xIP Q  M X 2
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QCD factorisation
get PDFs from inclusive diffraction ⇨ predict cross sections for exclusive diffraction
inclusive
dijet
hard scattering QCD
matrix element, perturbatively
calculated, process dependent
Universal diffractive parton densities
identical for all processes
 ( p  Xp ) 
D
*

f i ( x, Q 2 , x IP , t )    *i ( x, Q 2 )
D
parton _ i
  *i universal hard scattering cross section (same as in inclusive DIS)
D
fi diffractive parton distribution functions → obey DGLAP
universal for diffractive ep DIS (inclusive, di-jets, charm)
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Results from inclusive diffraction
Regge factorisation is an additional assumption, β
there is no PROOF!!
D
IP
fi ( x, Q2 , xIP , t )  f IP / p ( xIP , t )  fi (   x / xIP , Q2 )
pomeron flux factor
Q2
pomeron PDF
σdiff = flux(xP) · object (β,Q2)
Reduced cross section from
inclusive 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 GeV2 –1600 GeV2
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Low x meeting, Sinai 2005
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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 pomeron momentum
• large uncertainty at large zP
Assuming factorisation holds,
the jet and HQ cross sections give
better constraint on the gluon density
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Low x meeting, Sinai 2005
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Jet and HQ production
Hard scale is ET of the jet or HQ mass
Can reconstruct zP in dijet events
Direct access to gluon
density
• tests of universality of PDF’s
(=QCD factorisation)
• test of DGLAP evolution
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Low x meeting, Sinai 2005
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Charm cross section (DIS)
Good agreement within experimental & theoretical
uncertainties.
NLO calculations with PDFs from inclusive diffraction
NLO calculations HVQDIS
(Harris & Smith)
Good description of diffractive D* production in DIS
(2GeV2 <Q2<100 GeV2)
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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 H1
Hadronisation 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 with
NLO within exp.&theor.
uncertainties
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γ*p

pp
At Tevatron HERA PDF’s
do not work….????
Exporting PDFs from HERA to
the Tevatron.........
CDF Tevatron data:
Dijet cross section factor 5-10 lower
than the QCD calculation using HERA
PDFs
?
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Low x meeting, Sinai 2005
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Direct and resolved processes at HERA
xγ - fraction of photon’s momentum
in hard subprocess
x  x
OBS
(E  p )


z
jets
( E  pz )hadrons
DIS (Q2>5GeV2) and direct photoproduction (Q2≃0):
• photon directly involved in hard scattering
• xγ=1
unsuppressed!
?
Resolved photoproduction:
• photon fluctuates into hadronic system, which
takes part in hadronic scattering
• dominant at Q2≃0
• xγ<1
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suppressed!
?
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Photoproduction as hadronic process
HERA resolved photoproduction
Secondary interactions
between spectators
Typical models that describe suppression at Tevatron assume secondary
interactions of spectators as the cause:
resolved contribution expected to be suppressed by factor 0.34
(Kaidalov,Khoze,Martin,Ryskin:Phys.Lett.B567 (2003),61)
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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?
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