Ultra Peripheral Collisions

1. Au Au 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 .. STAR Collaboration Meeting, UCD group meeting, February 2008

2. 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/, o) emerges • Photon and pomeron are emitted coherently • Coherence condition limits transverse momentum of produced  Courtesy of F. Meissner STAR Collaboration Meeting, UCD group meeting, February 2008

3. Au* Au g (2+g) r0 P Au Au* ro Production With Coulomb Excitation Au+AuAu*+Au*+ro • Coulomb Excitation • Photons exchanged between ions give rise to excitation and subsequent neutron emission • Process is independent of o production Courtesy of S. Klein STAR Collaboration Meeting, UCD group meeting, February 2008

4. Courtesy of S. Klein Interference Nucleus 2 emits photon which scatters from Nucleus 1 Nucleus 1 emits photon which scatters from Nucleus 2 -Or- • 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: STAR Collaboration Meeting, UCD group meeting, February 2008

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

6. 1 neutron peak ! Triggers Au+AuAu+Au+ro 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 Au+AuAu*+Au*+ro 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 STAR Collaboration Meeting, UCD group meeting, February 2008

7. Studying the Interference • Determine  candidates by applying cuts to the data STAR Collaboration Meeting, UCD group meeting, February 2008

8. Studying the Interference p3 p1 p2 • t is a natural variable to use since it is invariant • can parameterize the spectrum ~ ebt • for our purposes p4 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. Studying the Interference • Generate MC ratio • Fit MC ratio STAR Collaboration Meeting, UCD group meeting, February 2008

11. C = 1.034±0.131 C = 0 Measuring the Interference • A= overall normalization • k = exponential slope • c = degree of interference • Apply overall fit c = 1 expected degree of interference c = 0 no interference C = 1.034±0.131 STAR Collaboration Meeting, UCD group meeting, February 2008

12. Results Summary STAR Collaboration Meeting, UCD group meeting, February 2008