HERA and the LHC

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HERA and the LHC
Diffractive Gap Probability
K. Goulianos, The Rockefeller University CERN
26-27 March 2004
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Diffractive signatures

• Leading hadron(s)
• Rapidity gap(s)

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Leading hadrons
Diffraction dissociation
KG, Phys. Rep. 101 (1983) 171
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Rapidity gaps

• Minimum bias

• Diffraction dissociation

From Poisson statistics
Gaps exponentially suppressed
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QCD aspects of diffraction

• Quark/gluon exchange across a rapidity gap
• 
  POMERON
• No particles radiated in the gap
• the exchange is
  COLOR-SINGLET with vacuum quantum numbers
• Rapidity gap formation
• 
  NON-PERTURBATIVE
• Diffraction probes the large distance aspects
  of QCD
• POMERON
  CONFINEMENT

• PARTONIC STRUCTURE
• FACTORIZATION PROPERTIES

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History of Diffraction
40 years of diffraction

• 1960 Good and Walker
• 1960s BNL first observation
• 1970s Fermilab fixed target, ISR, SPS
• 1983 KG, Phys. Rep. 101, 169 (1983)
• 1992 UA8 diffractive dijets a hard diff
• 1993-2003 Golden decade HERA, Tevatron, RHIC

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Soft and Hard Diffraction

• SOFT DIFFRACTION

• HARD DIFFRACTION

xDPL/PL xfractional momentum loss of
scattered (anti)proton
Additional variables (x, Q2)
Variables (x, t) or (Dh, t)
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Breakdown of QCD factorization
HERA
TEVATRON
p
x,t
IP
g
H1
e
CDF
gap
gap
dN/dh
dN/dh
h
h
XXX
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Diffractive vs inclusive structure fns
FDJJ(x,Q2) /F(x,Q2)
F2D(x,Q2) /F2(x,Q2)
no Q2 dependence flat at HERA 1/x0.5 at
Tevatron
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CDF Run I results
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Soft Diffraction

• Unitarity problem
• With factorization
• and std pomeron flux
• sSD exceeds sT at
• Renormalization
• normalize the pomeron
• flux to unity

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KG, PLB 358 (1995) 379
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A Scaling Law in Diffraction
KGJM, PRD 59 (1999) 114017
Factorization breaks down in favor of M2-scaling
renormalization
1
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Central and Double Gaps

• Double Diffraction Dissociation
• One central gap

• Double Pomeron Exchange
  
• Two forward gaps

• SDD SingleDouble Diffraction
• Forward central gaps

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Elastic total s
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Generalized renormalization (KG,
hep-ph/0205141)
t
2 independent variables
Gap probability
Renormalization removes the s-dependence
SCALING
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The factors k and e
Experimentally
KGJM, PRD 59 (114017) 1999
Color factor
Intercept
wggluon fraction wqquark fraction
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Two-Gap Diffraction (KG, hep-ph/0205141)
5 independent variables
color factor
Gap probability
Sub-energy cross section (for regions with
particles)
Same suppression as for single gap!
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Central Double-Gap Results
Differential shapes agree with Regge predictions
DD
SDD
DPE

• One-gap cross sections are suppressed

• Two-gap/one-gap ratios are

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Soft Double Pomeron Exchange

• Roman Pot triggered events
• 0.035 lt x-pbar lt 0.095
• t-pbar lt 1 GeV 2
• x-proton measured using
• Data compared to MC based
• on Pomeron exchange with ?
  Pomeron intercept e0.1

• Good agreement over 4 orders of magnitude!

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Soft gap survival probability
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Hard Diffraction QCD
Diffractive structure function

• The diffractive structure function measured using
  SD dijets at the Tevatron is suppressed by about
  an order of magnitude relative to predictions
  based on diffractive DIS at HERA
• The discrepancy is generally attributed to
  additional color exchanges which spoil the
  diffractive rapidity gap.

HERA
Factorization Breakdown
Tevatron
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Double-Gap Hard Diffraction _at_TEV vs HERA
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Diffractive structure functionfrom inclusive
pdfs (KG)
eg0.20
Power-law region xmax 0.1 xmax 0.1 b lt 0.05x
eq0. 04
eR-0.5
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Pomeron Intercept from H1
1l
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Pomeron intercept from ZEUS
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Gap between jets
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Exclusive Dijets in DPE
Interest in diffractive Higgs production
Calibrate on exclusive dijets
Dijet mass fraction
no peak!
Upper limit for excl DPE-jj consistent with theory
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Exclusive cc Production in DPE
Cross section upper limit comparable to KMR
prediction
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Summary
SOFT DIFFRACTION

• M2 scaling
• Non-suppressed double-gap to single-gap ratios
• Flavor-independence SD/ND ratio
• Get diffractive from non-diffractive pdfs

HARD DIFFRACTION
Universality of gap prob. across soft and hard
diffraction
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HERA Tevatron-gtLHC