STAR Collaboration Meeting, UCD group meeting, February 2008

1

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

• Weizsacker-Williams

Equivalent Photon Approximation

• No hadronic interactions

..

Au

Au

€

d3N(k,r)

dkd 2r=

Z 2α x 2

π 2kr2K1

2(x)

€

x =kr

γ

STAR Collaboration Meeting, UCD group meeting, February 2008

2

Exclusive o Production

• Photon emitted by a nucleus fluctuates to virtual qq pair

• Virtual qq pair elastically scatters from other nucleus

• Real vector meson (i.e. J/, o) emerges

• Photon and pomeron are emitted coherently

• Coherence condition limits transverse momentum of produced

Courtesy of F. Meissner

Au+Au Au+Au+o

€

pT <h

2RA

STAR Collaboration Meeting, UCD group meeting, February 2008

3

o Production With Coulomb Excitation

Au+Au Au*+Au*+o

Au

Au

P

Au*

Au*

(2+)0

Courtesy of S. Klein

• Coulomb Excitation• Photons exchanged between

ions give rise to excitation and subsequent neutron emission

• Process is independent of o production

€

σ(AuAu → Au*Au* + ρ o) = d2bP∫ρ(b)PXnXn (b)

STAR Collaboration Meeting, UCD group meeting, February 2008

4

Courtesy of S. Klein

Nucleus 1 emits photon which scatters from Nucleus 2

Nucleus 2 emits photon which scatters from Nucleus 1-Or-

Interference

• Amplitude for observing vector meson at a distant point is the 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[φ(y )+

r p •(

r x −

r x o )] − A(p⊥,−y,b)e i[φ(−y )+

r p •(

r x −

r x o )]

€

σ =A2( p⊥, y,b) + A2(p⊥,y,b) − 2A(p⊥, y,b)A(p⊥,−y,b) × cos[φ(y) − φ(−y) +r p •

r b ]

€

σ =2A2(p⊥,b)(1− cos[r p •

r b ])

STAR Collaboration Meeting, UCD group meeting, February 2008

5

Central Trigger Barrel

Time Projection Chamber

STAR Analysis Detectors

Zero Degree

Calorimeter

Zero Degree

Calorimeter

STAR Collaboration Meeting, UCD group meeting, February 2008

6

UPC Topology• Central Trigger Barrel divided into four quadrants• Verification of decay candidate with hits in North/South quadrants• Cosmic Ray Background vetoed in Top/Bottom quadrants

Au+Au Au+Au+o

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

photonuclearinteractions

Au+Au Au*+Au*+o

1 neutron peak !

STAR Collaboration Meeting, UCD group meeting, February 2008

7

Studying the Interference

• Determine 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

8

Studying the Interference

€

s = (p1 + p2)2

t = (p1 − p3)2

u = (p1 − p4 )2 p1 p2

p3

p4

€

t ∝ pT2

• 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

9

Studying the Interference

• Generate similar MC histograms

STAR Collaboration Meeting, UCD group meeting, February 2008

10

€

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, UCD group meeting, February 2008

11

Measuring the Interference

• Apply overall fit

€

dN

dt= Ae−kt (1− c[R(t) −1])

c = 1 expected 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, UCD group meeting, February 2008

12

Results Summary

c 2/dof

Minbias

0.0 < y < 0.5 0.92±0.07

45/47

0.5 < y < 1.0 0.920.09

76/47

Topology

0.05 < y < 0.5 0.73±0.10

53/47

0.5 < y < 1.0 0.770.18

64/47