interference in coherent vector meson production in upc au...
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STAR Collaboration Meeting, MIT, July 2006 1
Interference in CoherentVector Meson Production in
UPC Au+Au Collisions at√s = 200GeV
Brooke HaagUC Davis
STAR Collaboration Meeting, MIT, July 2006 2
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 3
Ultra Peripheral Collisions
• What is a UPC?• Photonuclear interaction• Two nuclei “miss” each other (b > 2RA), electromagnetic
interaction dominates overstrong interaction
• Photon flux ~ Z2
• Weizsacker-WilliamsEquivalent PhotonApproximation
• No hadronic interactions
..
Au
Au
!
d3N(k,r)
dkd2r
=Z2"x 2
# 2kr
2K1
2(x)
!
x =kr
"
STAR Collaboration Meeting, MIT, July 2006 4
Exclusive ρo Production
• Photon emitted by a nucleusfluctuates to virtual qq pair
• Virtual qq pair elastically scattersfrom other nucleus
• Real vector meson (i.e. J/ψ, ρo)emerges
• Photon and pomeron are emittedcoherently
• Coherence condition limitstransverse momentum ofproduced ρ
Courtesy of F. Meissner
Au+Au Au+Au+ρo
!
pT <h
2RA
STAR Collaboration Meeting, MIT, July 2006 5
ρo Production With CoulombExcitation
Au+Au Au*+Au*+ρo
Au
Au
γ
PAu*
Au*
(2+γ)ρ0
Courtesy of S. Klein
• Coulomb Excitation• Photons exchanged between
ions give rise to excitation andsubsequent neutron emission
• Process is independent of ρo
production
!
"(AuAu# Au*Au
*+ $o
) = d2bP%
$(b)P
XnXn(b)
STAR Collaboration Meeting, MIT, July 2006 6
Courtesy of S. Klein
Nucleus 1 emits photon whichscatters from Nucleus 2
Nucleus 2 emits photon whichscatters from Nucleus 1-Or-
Interference
• Amplitude for observing vector meson at a distant point isthe convolution of two plane waves:
• Cross section comes from square of amplitude:
• We can simplify the expression if y → 0:
!
Ao(xo,r p ,b) = A(p",y,b)e
i[# (y )+r p •(
r x $
r x o )] $ A(p",$y,b)e
i[# ($y )+r p •(
r x $
r x o )]
!
" = A2(p#,y,b) + A
2(p#,y,b) $ 2A(p#,y,b)A(p#,$y,b) % cos[&(y) $&($y) +
r p •
r b ]
!
" = 2A2(p#,b)(1$ cos[
r p •
r b ])
STAR Collaboration Meeting, MIT, July 2006 7
CentralTriggerBarrel
TimeProjectionChamber
STAR Analysis Detectors
Zero Degree
Calorimeter
Zero Degree
Calorimeter
STAR Collaboration Meeting, MIT, July 2006 8
UPC Topology• Central Trigger Barrel divided into four quadrants• Verification of ρ decay candidate with hits in
North/South quadrants• Cosmic Ray Background vetoed in Top/Bottom quadrants
Au+Au Au+Au+ρo
Triggers
UPC Minbias• 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
Au+Au Au*+Au*+ρo
STAR Collaboration Meeting, MIT, July 2006 9
Studying the Interference
• Determine ρο candidates by applying cuts to the data
> 0 GeV< 0.1 GeV
pT
> 0.55 GeV< 0.92 GeV
MInv
> 0.1< 0.5
rapidity
< 8 cm|rVertex|
< 50 cm|zVertex|
2nPrim
2nTot
0qTot
STAR Collaboration Meeting, MIT, July 2006 10
Studying the Interference
• Generate similar MC histograms
STAR Collaboration Meeting, MIT, July 2006 11
!
R(t) =Interference(t)
No interference(t)
Studying the Interference
• Generate MC ratio• Fit MC ratio
!
R(t) = a +b
(t + 0.012)+
c
(t + 0.012)2
+d
(t + 0.012)3
+e
(t + 0.012)4
STAR Collaboration Meeting, MIT, July 2006 12
Measuring the Interference
• Apply overall fit
!
dN
dt= Ae
"kt(1" cR(t))
c = 1expected degree of
interference
c = 0 no interference
C = 1.034±0.131
• A= overall normalization• k = exponential slope• c = degree of interference
C = 0C = 1.034±0.131
STAR Collaboration Meeting, MIT, July 2006 13
Results
Topology
C = 0.8487±0.1192
χ2/DOF = 87.92/47
Minbias
C = 1.009±0.081
χ2/DOF = 50.77/47
Au+Au Au*+Au*+ρo Au+Au Au+Au+ρo
STAR Collaboration Meeting, MIT, July 2006 14
Results Summary
83.81/47
87.92/47
80.18/47
50.77/47
χ2/dof
1.059±0.208
0.5 < y < 1.0
Topology
0.8487±0.1192
0.1 < y < 0.5
0.9275±0.1095
0.5 < y < 1.0
1.009±0.081
0.1 < y < 0.5
Minbias
c
STAR Collaboration Meeting, MIT, July 2006 15
Interference routine for minbias 0 > y > 0.5
• flat ratio
~10%
• statistics
STAR Collaboration Meeting, MIT, July 2006 16
Systematic Error Study
-0.414955topology
0.09352014minbias0 < y < 0.50.1 < y < 0.5rapidity
0.0454628minbiaszVertex < 0|zVertex| < 500.5 < y < 1.0
0.1526637minbiaszVertex > 0|zVertex| < 500.5 < y < 1.0
-0.3231100topology
0.0379
0.1188
0.1883
0.0422
Uncertainty
1844topology
826minbiaszVertex < 0|zVertex| < 500.1 < y < 0.5
1989topology
811minbiaszVertex > 0|zVertex| < 500.1 < y < 0.5
zVertex
EntriesData SetVaried CutStandard Cut
STAR Collaboration Meeting, MIT, July 2006 17
Systematic Error Study
0.9%
1.3%
0.008topology
0.013minbias6 parameter
UncertaintyData SetFit
The 5 parameter fit is sufficient -- adding anotherparameter doesn’t improve the analysis.
!
R(t) = a +b
(t + 0.012)+
c
(t + 0.012)2
+d
(t + 0.012)3
+e
(t + 0.012)4
!
R(t) = a +b
(t + 0.012)+
c
(t + 0.012)2
+d
(t + 0.012)3
+e
(t + 0.012)4
+f
(t + 0.012)5
STAR Collaboration Meeting, MIT, July 2006 18
Paper Proposal
http://www.star.bnl.gov/protected/pcoll/bhaag/NewPage/Frames.html
STAR Collaboration Meeting, MIT, July 2006 19
Backup Slides
STAR Collaboration Meeting, MIT, July 2006 20Topology rapidity study
• two rapidityranges
|y| < 0.05
|y| < 0.1
• rVertex Tight =rVertex < 8 cm
• rVertex Loose =rVertex < 16 cm
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