Diffraction at DØ
Andrew Brandt
University of Texas, Arlington
E
f
h
• Intro and Run I Hard Diffraction Results
• Run II and Forward Proton Detector
Physics Colloquium
November 5, 2003
UTA
Fermilab Tevatron
Chicago
CDF
DØ
DØ
CDF
DØ
Booster
Tevatron
p source
p
p
Main Injector
& Recycler
p
p
Run I ( 1992 - 1995 ) : s =1.8 TeV
Run Ic (1995):
s =630 GeV
Run II ( 2001 - ? ) :
s =1.96 TeV
Proton
remnant
spectator partons
p
f
p
Jet
h
Elastic Scattering
• The particles after scattering are the same
as the incident particles
• The cross section can be written as:
d dt
 ebt  1  b( p ) 2
d dt t  0
• 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).
Diffraction at Tevatron
pppp
p p  p (p) + X
p p  p (p) + j j
ppp p+jj
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 pp ?
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
d 2
dtdx
without FPD just measure 
Diffractive Variables
pBeam
pF
P
t  ( PBeam  PF ) 2
P
x  1 xp 
P
Mx  x s
Pomeron Exchange
Non-diffractive
s  2 TeV
For
For pp  p  X
pp  pp  X
(Note:
ep  ep  X
x p  0.95
x  0.05
M x  450 GeV
M x  100 GeV
M x  70 GeV )
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
A*
vacuum
A
P
J2
X
J1
B
G. Ingelman and P. Schlein, Phys. Lett. B 152, 256 (1985)
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
 ( PB  X )    dbdxb f a / P (b )
ab
 f b / B ( xb )ˆ (ab  X )
• 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
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)
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 (nonperturbative) interactions can change the
hadronization process such that color is
bleached out and rapidity gaps appear
• Soft Color shows same exponential tdependence as Ingelman-Schlein due to
primordial kT of the partons
• Suggests a formation rate of gaps in gluongluon sub processes which is less than or
equal to the formation rate in quark-quark
sub processes
Color Evaporation II
f
h
f
h
f
h
R. Enberg, G. Ingelman, and N. Timneanu, Phys. Rev. D 64,
114015 (2001).
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
40 years of Diffraction
60’s: First evidence for hadronic diffraction, S matrix
Regge theory, Pomeron
70’s: DIS, High pT processes. Parton model, QCD,
c, τ, b, gluon
80’s: Ingelman-Schlein, BFKL
90’s: Hard Diffraction (UA8), Rapidity Gaps (Bjorken),
HERA (diffraction in ep), Tevatron
DØ Detector (Run I)
(nl0 = # tiles in L0 detector with signal
2.3 < |h| < 4.3)
beam
L0 Detector
End Calorimeter
Central Calorimeter
EM Calorimeter
Central Drift Chamber
(ntrk = # charged tracks with
|h| < 1.0)
Hadronic Calorimeter
(ncal = # cal towers with energy above threshold)
Central Gaps
EM Calorimeter
ET > 200 MeV
|h| < 1.0
EM Calorimeter
E > 150 MeV
2.0 < |h| < 4.1
Had. Calorimeter
E > 500 MeV
3.2 < |h| < 5.2)
Forward Gaps
Hard Color-Singlet Studies
QCD color-singlet f
signal observed in
~ 1 % opposite-side
events (p p )
h
jet
jet
Publications
h
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:
h, ET, s (parton-x)
Jill Perkins (UTA), Ph. D. on color singlet at s = 630
Hard Color-Singlet Exchange
Count tracks and EM
Calorimeter Towers in
|h| < 1.0
f
h
jet
jet
h
(ET > 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)
1800 and 630 GeV Single
Diffractive Multiplicities
s = 1800 GeV
s = 630 GeV
Peak at zero multiplicity striking indication of rapidity gap (diffractive)
signature in events with hard scattering (two jets with ET>12 GeV)
B. Abbott et al. (DØ Collaboration), Phys. Lett. B 531, 52 (2002)
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
P
x = 1 - xp
(momentum loss of proton)
•Data gap fractions ~1%, higher for s=630 than 1800 GeV
•Data fraction < MC hard gluon or quark fractions (normalization
difference), ratios imply this is not a simple normalization
factor—would require significant soft gluon structure to
save Ingelman-Schlein type model; rates favor SCI model
Double Gaps at 1800 GeV
|Jet h| < 1.0, ET>15 GeV
Gap Region
2.5<|h|<5.2
Demand gap on one side, measure multiplicity on opposite side
DØ Preliminary
Double Gaps at 630 GeV
|Jet h| < 1.0, ET>12 GeV
Gap Region
2.5<|h|<5.2
Demand gap on one side, measure multiplicity on opposite side
DØ Preliminary