
Ultra Peripheral Collisions
• What is a UPC?
• Photonuclear interaction
• Two nuclei “miss” each other
(b > 2RA), electromagnetic
interaction dominates over strong
interaction
• Photon flux ~ Z2
..
Au
• Weizsacker-Williams
Equivalent Photon Approximation
d 3 N(k,r) Z 2 x 2 2
 2 2 K1 (x)
2
dkd r
 kr
• No hadronic interactions
x
kr


STAR Collaboration Meeting, UCD group meeting, February
2008
Au
1
Exclusive ro Production
Au+Au  Au+Au+ro
• Photon emitted by a nucleus
fluctuates to virtual qq pair
• Virtual qq pair elastically scatters
from other nucleus
• Real vector meson (i.e. J/, ro)
emerges
• Photon and pomeron are emitted
coherently
• Coherence condition limits
transverse momentum of produced r
h
pT 
2RA
STAR Collaboration Meeting, UCD group meeting, February
2008
2
ro Production With Coulomb Excitation
Au+Au  Au*+Au*+ro
Au
Au*

r0
(2+)
P
• Coulomb Excitation
• Photons exchanged between ions
give rise to excitation and
subsequent neutron emission
• Process is independent of ro
production
Au
 (AuAu  Au* Au*  r o ) 
Au*
 d bP r (b)P
2
XnXn
Courtesy of S. Klein

STAR Collaboration Meeting, UCD group meeting, February
2008
3
(b)
Interference
Nucleus 1 emits photon which
scatters from Nucleus 2
-Or-
Nucleus 2 emits photon which
scatters from Nucleus 1
Courtesy of S. Klein
• Amplitude for observing vector meson at a distant point is the
convolution of two plane waves:
Ao (x o, p,b)  A( p , y,b)e i[ (y ) p (x xo )]  A( p ,y,b)e i[ (y ) p (x xo )]
• Cross section comes from square of amplitude:
  A2 ( p , y,b)  A2 (p , y,b)  2A(p , y,b)A(p ,y,b)  cos[(y)  (y)  p b]
• We can simplify the expression if y  0:
  2A2 (p ,b)(1 cos[ p b])
STAR Collaboration Meeting, UCD group meeting, February
2008
4
STAR Analysis Detectors
Time
Projection
Chamber
Zero Degree
Zero Degree
Calorimeter
Calorimeter
Central
Trigger
Barrel
STAR Collaboration Meeting, UCD group meeting, February
2008
5
Triggers
UPC Topology
•
•
•
Central Trigger Barrel divided into four quadrants
Verification of r decay candidate with hits in
North/South quadrants
1 neutron peak
Cosmic Ray Background vetoed in Top/Bottom quadrants
UPC Minbias
•
•
•
Au+Au  Au+Au+ro
!
Au+Au  Au*+Au*+ro
Minimum one neutron in each Zero Degree
Calorimeter required
Low Multiplicity
Not Hadronic Minbias!
Trigger Backgrounds
• Cosmic Rays
• Beam-Gas interactions
• Peripheral hadronic
interactions
• Incoherent photonuclear
interactions
STAR Collaboration Meeting, UCD group meeting, February
2008
6
Studying the Interference
• Determine r candidates by applying cuts to the data
qTot
0
nTot
2
nPrim
2
|zVertex|
< 50 cm
|rVertex|
< 8 cm
rapidity
> 0.1
< 0.5
MInv
> 0.55 GeV
< 0.92 GeV
pT
> 0 GeV
< 0.1 GeV
STAR Collaboration Meeting, UCD group meeting, February
2008
7
Studying the Interference
s  ( p1  p2 ) 2
p3
t  ( p1  p3 ) 2
u  ( p1  p4 ) 2
p1
p2
p4
• t is a natural variable to
use since it is invariant
• can parameterize the
spectrum ~ ebt
• for our purposes
STAR Collaboration Meeting, UCD group meeting, February
2008
t  pT2
8
Studying the Interference
•
Generate similar MC histograms
STAR Collaboration Meeting, UCD group meeting, February
2008
9
Studying the Interference
• Generate MC ratio
• Fit MC ratio
R(t) 
Interference(t)
No interference(t)

R(t)  a 
b
c
d
e



(t  0.012) (t  0.012) 2 (t  0.012) 3 (t  0.012) 4
STAR Collaboration Meeting, UCD group meeting, February
2008
10
Measuring the Interference
• Apply overall fit
dN
 Aekt (1 c[R(t) 1])
dt

C = 1.034±0.131
C = 1.034±0.131
C=0
• A= overall normalization
• k = exponential slope
• c = degree of interference
c=1
expected degree of
interference
c=0
no interference
STAR Collaboration Meeting, UCD group meeting, February
2008
11
Results Summary
c
2/dof
Minbias
0.0 < y < 0.5
0.92±
0.07
45/47
0.5 < y < 1.0
0.92 0.09
76/47
0.05 < y < 0.5
0.73±
0.10
53/47
0.5 < y < 1.0
0.77 0.18
64/47
Topology
STAR Collaboration Meeting, UCD group meeting, February
2008
12