Interference in Coherent
Vector Meson Production in
UPC Au+Au Collisions at
√s = 200GeV
Brooke Haag
UC Davis
STAR Collaboration Meeting, MIT, July 2006
1
Outline
• Ultra Peripheral Heavy Ion Collisions (UPCs)
• What is a UPC?
• Vector Meson Production / Interference
• STAR detectors / Triggers
• Analysis of UPC events
• Fitting Scheme
• Observation of interference effects in t spectrum
• Systematic Errors and Outlook
STAR Collaboration Meeting, MIT, July 2006
2

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, MIT, July 2006
Au
3
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
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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, MIT, July 2006
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(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, MIT, July 2006
6
STAR Analysis Detectors
Time
Projection
Chamber
Zero Degree
Zero Degree
Calorimeter
Calorimeter
Central
Trigger
Barrel
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Triggers
UPC Topology
•
•
•
Central Trigger Barrel divided into four quadrants
Verification of r decay candidate with hits in
North/South quadrants
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, MIT, July 2006
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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
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Studying the Interference
•
Generate similar MC histograms
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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, MIT, July 2006
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Measuring the Interference
• Apply overall fit
dN
 Aekt (1 cR(t))
dt

• A= overall normalization
• k = exponential slope
• c = degree of interference
C = 1.034±0.131
C = 1.034±0.131
C=0
c=1
expected degree of
interference
c=0
no interference
STAR Collaboration Meeting, MIT, July 2006
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Results
Minbias
Au+Au  Au*+Au*+ro
Topology
Au+Au  Au+Au+ro
C = 1.009±0.081
C = 0.8487±0.1192
2/DOF = 50.77/47
2/DOF = 87.92/47
STAR Collaboration Meeting, MIT, July 2006
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Results Summary
c
2/dof
Minbias
0.1 < y < 0.5
1.009±
0.081
50.77/47
0.5 < y < 1.0
0.9275 0.
1095
80.18/47
0.1 < y < 0.5
0.8487±
0.1192
87.92/47
0.5 < y < 1.0
1.059 0.2
08
83.81/47
Topology
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Interference routine for minbias 0 > y > 0.5
~10%
• flat ratio
• statistics
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Systematic Error Study
zVertex
Standard Cut
Varied Cut
Data Set
Entries
Uncertainty
|zVertex| < 50
0.1 < y < 0.5
zVertex > 0
minbias
811
0.0422
topology
1989
0.1883
minbias
637
0.1526
topology
1100
-0.323
minbias
826
0.1188
topology
1844
0.0379
minbias
628
0.0454
topology
955
-0.414
minbias
2014
0.093516
|zVertex| < 50
0.5 < y < 1.0
|zVertex| < 50
0.1 < y < 0.5
|zVertex| < 50
0.5 < y < 1.0
rapidity
0.1 < y < 0.5
zVertex > 0
zVertex < 0
zVertex < 0
0 < y < 0.5
STAR Collaboration Meeting, MIT, July 2006
Systematic Error Study
R(t)  a 

R(t)  a 
b
c
d
e



(t  0.012) (t  0.012) 2 (t  0.012) 3 (t  0.012) 4
b
c
d
e
f




(t  0.012) (t  0.012) 2 (t  0.012) 3 (t  0.012) 4 (t  0.012) 5
Fit
Data Set
Uncertainty
6 parameter
minbias
0.013
1.3%
topology
0.008
0.9%

The 5 parameter fit is sufficient -- adding another
parameter doesn’t improve the analysis.
STAR Collaboration Meeting, MIT, July 2006
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Paper Proposal
http://www.star.bnl.gov/protected/pcoll/bhaag/NewPage/Frames.html
STAR Collaboration Meeting, MIT, July 2006
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