Coherent Interactions in Nucleus-Nucleus Collisions

At the Relativistic Heavy-Ion Collider (RHIC) at Brookhaven National Laboratory and at the Large Hadron Collider (LHC) at CERN, particles will be produced in coherent and diffractive nuclear interactions. In extremely peripheral nuclear collisions (b$>$2R), coherent interactions occur at very high rates and are dominated by photon-Pomeron or photon-meson processes. In these reactions, the photon and the Pomeron/meson from the electromagnetic and nuclear fields couple coherently to all nucleons. The rates for photonuclear interactions are roughly two orders of magnitude larger than for two-photon interactions at comparable center-of-mass energies.Comment: 10 pages, 4 figures, Presented at the IXth Blois Workshop on Elastic and Diffractive Scattering, Pruhonice near Prague, Czech Republic, June 9-15, 200

Photonuclear vector meson production in ultra-peripheral Pb-Pb collisions studied by the ALICE experiment at the LHC

The strong electromagnetic fields surrounding the Pb-ions accelerated at the CERN Large Hadron Collider (LHC) allow two-photon and photonuclear interactions to be studied in a so far unexplored kinematic regime. Exclusive photoproduction of vector mesons can be studied in ultra-peripheral collisions, where the impact parameters are larger than the sum of the nuclear radii and hadronic interactions are strongly suppressed. During the heavy-ion runs at the LHC in 2010 and 2011, the ALICE collaboration used special triggers to select ultra-peripheral collisions. These triggers were based on the Muon spectrometer, the Time-of-Flight detector, the Silicon Pixel detector, and the VZERO scintillator array. Information from other detectors was also used in the analysis. The cross section for coherent photoproduction of J/Psi mesons at forward rapidities will be presented. The result will be compared to model calculations and its implications for nuclear gluon shadowing will be discussed.Comment: 7 pages, 3 figures. Presented at the Xth Quark Confinement and the Hadron Spectrum Conference, Munich, Germany, 8 - 12 October 201

Photonuclear production of vector mesons in ultra-peripheral Pb-Pb collisions at the LHC

Vector mesons are copiously produced in ultra-peripheral nucleus-nucleus collisions. In these collisions, the nuclei are separated by impact parameters larger than the sum of the nuclear radii, and the interaction is mediated by the electromagnetic field. The interaction effectively corresponds to a photonuclear interaction between a photon, generated from the electromagnetic field of one of the nuclei, and the target nucleus. The ALICE Collaboration has previously published results on exclusive J/psi photoproduction at mid and forward rapidities in Pb-Pb collisions. The cross section for this process is a particularly good measure of the nuclear gluon distribution. In this talk, the latest results on exclusive production of light and heavy vector mesons from ALICE in Pb-Pb collisions will be presented.Comment: 4 pages, 3 figures. Presented at the XXIV International Conference on Ultrarelativistic Nucleus-Nucleus Collisions (Quark Matter 2014), Darmstadt, Germany, 19-24 May 2014. Fixed typo in Eq.

Two-Photon Physics at RHIC: Separating Signals from Backgrounds

This presentation will show the feasibility of studying two-photon interactions in the STAR experiment at RHIC. Signals, detection efficiencies, backgrounds, triggering and analysis techniques will be discussed.Comment: 5 pages, 1 figure, 3 table