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