1. STAR Collaboration Meeting, UCD group meeting, February 2008 1 Ultra Peripheral Collisions What is a UPC? Photonuclear interaction Two nuclei “miss” each other (b > 2R A ), electromagnetic interaction dominates over strong interaction Photon flux ~ Z 2 Weizsacker-Williams Equivalent Photon Approximation No hadronic interactions.. Au

2. 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

3. STAR Collaboration Meeting, UCD group meeting, February 2008 3  o Production With Coulomb Excitation Au+Au  Au*+Au*+  o Au  P Au* ( 2+  ) 0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

4. 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:

5. STAR Collaboration Meeting, UCD group meeting, February 2008 5 Central Trigger Barrel Time Projection Chamber STAR Analysis Detectors Zero Degree Calorimeter Zero Degree Calorimeter

6. 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 photonuclear interactions Au+Au  Au*+Au*+  o 1 neutron peak !

7. STAR Collaboration Meeting, UCD group meeting, February 2008 7 Studying the Interference Determine   candidates by applying cuts to the data qTot0 nTot2 nPrim2 |zVertex|< 50 cm |rVertex|< 8 cm rapidity> 0.1 < 0.5 M Inv > 0.55 GeV < 0.92 GeV pTpT > 0 GeV < 0.1 GeV

8. STAR Collaboration Meeting, UCD group meeting, February 2008 8 Studying the Interference p1p1 p2p2 p3p3 p4p4 t is a natural variable to use since it is invariant can parameterize the spectrum ~ e bt for our purposes

9. STAR Collaboration Meeting, UCD group meeting, February 2008 9 Studying the Interference Generate similar MC histograms

10. STAR Collaboration Meeting, UCD group meeting, February 2008 10 Studying the Interference Generate MC ratio Fit MC ratio

11. STAR Collaboration Meeting, UCD group meeting, February 2008 11 Measuring the Interference Apply overall fit 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 = 0 C = 1.034±0.131

12. STAR Collaboration Meeting, UCD group meeting, February 2008 12 Results Summary c  2 /dof Minbias 0.0 < y < 0.50.92± 0.07 45/47 0.5 < y < 1.00.92  0.0976/47 Topology 0.05 < y < 0.50.73± 0.10 53/47 0.5 < y < 1.00.77  0.1864/47