O U T L I N E

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Diffraction at DØ
Andrew Brandt University of Texas, Arlington

• Intro and Run I Hard Diffraction Results
• Run II and Forward Proton Detector

Physics Colloquium November 5, 2003 UTA
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Fermilab Tevatron
Chicago
Booster
p
Tevatron
p
p source
Main Injector Recycler
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Proton remnant
spectator partons
p
?
?
Jet
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Elastic Scattering

• The particles after scattering are the same as
  the incident particles
• The cross section can be written as
• This has the same form as light diffracting from
  a small absorbing disk, hence the name
  diffractive phenomena

Elastic dip Structure from Phys. Rev. Lett.
54, 2180 (1985).
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Diffraction at Tevatron
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Learning about the Pomeron

• QCD is theory of strong interactions, but 40
  of
• total cross section is attributable to
  Pomeron
• exchange -- not calculable and poorly
  understood
• Does it have partonic structure?
• Soft? Hard? Super-hard? Quark? Gluon?
• Is it universal -- same in ep and ?
• Is it the same with and without jet
  production?
• Answer questions in HEP tradition -- collide it
  
• with something that you understand to learn
  
• its structure
• Note variables of diffraction are t and x
  M2
• with FPD measure
• without FPD just measure s

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Diffractive Variables
pBeam
pF
P
Pomeron Exchange
Non-diffractive
For TeV
For
GeV
GeV
(Note
)
GeV
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Ingelman-Schlein

• Factorization allows us to look at the
  diffractive reaction as a two step process.
  Hadron A emits a Pomeron (pomeron flux) then
  partons in the Pomeron interact with hadron B.
• The Pomeron to leading order is proposed to have
  a minimal structure of two gluons in order to
  have quantum numbers of the vacuum

A
A
P
J2
X
J1
B
G. Ingelman and P. Schlein, Phys. Lett. B 152,
256 (1985)
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Ingelman-Schlein II

• The flux factor term has been derived by
  Donnachie and Landshoff after comparison to
  global data
• The remaining cross-section can be found from
  standard factorization processes to be
• The only unknown is the structure function of
  parton a (with momentum fraction b) in the
  Pomeron so measurements of the cross section
  allow us to probe this structure function

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BFKL

• Balitsky, Fadin, Kuraev and Lipatov
• Starting with two reggeized gluons we can add
  perturbative corrections of real ladder gluons
  and virtual radiative gluons to get a gluon
  ladder
• Mathematically, each successive order of
  correction adds a power of log s to the
  perturbative expansion and at sufficient energies
  will break the perturbation
• BFKL Proposes to fix this by isolating in each
  order the contribution with the highest power of
  log s and resumming these leading terms (leading
  logarithmic approximation)

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Color Evaporation (SCI)

• This theory attempts to account for rapidity gaps
  in diffractive events without resorting to the
  use of a Pomeron
• Model has been successfully applied to onium
  production (charmonium, J/psi)
• Proposes that allowing soft color
  (non-perturbative) interactions can change the
  hadronization process such that color is bleached
  out and rapidity gaps appear
• Soft Color shows same exponential t-dependence as
  Ingelman-Schlein due to primordial kT of the
  partons
• Suggests a formation rate of gaps in gluon-gluon
  sub processes which is less than or equal to the
  formation rate in quark-quark sub processes

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Color Evaporation II
f

h
f
h
f
h
R. Enberg, G. Ingelman, and N. Timneanu, Phys.
Rev. D 64, 114015 (2001).
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Pomeron Structure
1) UA8 shows partonic structure of pomeron
(diffractive dijet production) consistent with
hard structure (like gg or qq) and perhaps a
super-hard component 2) HERA DIS with large gap
shows a quark component in pomeron, F2D
shows pomeron dominantly gluonic 3) HERA
diffractive jet and structure function
analysis indicate dominantly hard gluonic
structure 4) Comparison of HERA and Tevatron
data crucial for understanding pomeron
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40 years of Diffraction
60s First evidence for hadronic diffraction, S
matrix Regge theory, Pomeron
70s DIS, High pT processes. Parton model,
QCD, c, t, b, gluon
80s Ingelman-Schlein, BFKL
90s Hard Diffraction (UA8), Rapidity Gaps
(Bjorken), HERA (diffraction in ep), Tevatron
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DØ Detector (Run I)
(nl0 tiles in L0 detector with signal 2.3 lt
h lt 4.3)
beam
L0 Detector
End Calorimeter
Central Calorimeter
EM Calorimeter
Central Drift Chamber
(ntrk charged tracks with h lt 1.0)
Hadronic Calorimeter
(ncal cal towers with energy above threshold)
Central Gaps EM Calorimeter ET gt 200 MeV h
lt 1.0 Forward Gaps EM Calorimeter E gt 150
MeV 2.0 lt ? lt 4.1 Had. Calorimeter E gt 500
MeV 3.2 lt ? lt 5.2)
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Hard Color-Singlet Studies
QCD color-singlet signal observed in 1
opposite-side events (p )
f
Dh
jet
jet
h
Publications DØ PRL 72,
2332(1994) CDF PRL 74, 885 (1995) DØ
PRL 76, 734 (1996) Zeus Phys Lett B369, 55
(1996) (7) CDF PRL 80, 1156 (1998)
DØ PLB 440, 189 (1998) CDF PRL 81, 5278
(1998) H1 Eur.Phys.J. C24 517 (2002)

• Color-Singlet fractions at
• ?s 630 1800 GeV
• Color-Singlet Dependence on
• Dh, ET, ?s (parton-x)

Jill Perkins (UTA), Ph. D. on color singlet at ?s
630
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Hard Color-Singlet Exchange
f
Count tracks and EM Calorimeter Towers in h lt
1.0
Dh
jet
jet
h
(ET gt 30 GeV, ?s 1800 GeV)
Measure fraction of events due to color-singlet
exchange
Measured fraction (1) rises with initial quark
content Consistent with a soft color
rearrangement model preferring initial quark
states Inconsistent with two-gluon, photon, or
U(1) models
Phys. Lett. B 440 189 (1998)
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1800 and 630 GeV Single Diffractive Multiplicities
Peak at zero multiplicity striking indication of
rapidity gap (diffractive) signature in events
with hard scattering (two jets with ETgt12 GeV) B.
Abbott et al. (DØ Collaboration), Phys. Lett.
B 531, 52 (2002)
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POMPYT Monte Carlo
p p ? p (or p) j j

Model pomeron exchange POMPYT26
(Bruni Ingelman) based on PYTHIA
define pomeron as beam particle
P
p
Structure Functions 1) Hard Gluon
xG(x) x(1-x) 2) Flat Gluon (flat in x) 3)
Quark xG(x) x(1-x) 4) Soft Gluon xG(x)
(1-x)5
p
p
? 1 - xp (momentum loss of proton)
P
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• Data gap fractions 1, higher for ?s630 than
  1800 GeV
• Data fraction lt MC hard gluon or quark fractions
  (normalization
• difference), ratios imply this is not a simple
  normalization
• factorwould require significant soft gluon
  structure to
• save Ingelman-Schlein type model rates favor
  SCI model

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Double Gaps at 1800 GeVJet h lt 1.0, ETgt15 GeV
Gap Region 2.5lthlt5.2
Demand gap on one side, measure multiplicity on
opposite side
DØ Preliminary
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Double Gaps at 630 GeVJet h lt 1.0, ETgt12 GeV
Gap Region 2.5lthlt5.2
Demand gap on one side, measure multiplicity on
opposite side
DØ Preliminary