Photoproduction In Ultra-Peripheral Heavy Ion Collisions Research Articles

Exclusive Four-pion Photoproduction in Ultra-peripheral Heavy-ion Collisions at RHIC and LHC Energies

We study the photoproduction of exclusive $2\pi^+2\pi^-$ mesons in ultra-peripheral heavy-ion collisions at RHIC and LHC energies. Predictions in photon-nucleus interactions are calculated for various resonances at central and forward rapidities. The recent H1 preliminary data are utilized to improve the description of the poorly known $\gamma p \to 4\pi^\pm p$ process. We present the comparisons of our results to the available STAR data at RHIC, and made predictions for LHC energies.

Observation ofπ+π−π+π−photoproduction in ultraperipheral heavy-ion collisions atsNN=200GeV at the STAR detector

We present a measurement of π+π−π+π− photonuclear production in ultraperipheral Au−Au collisions at sNN=200 GeV from the STAR experiment. The π+π−π+π− final states are observed at low transverse momentum and are accompanied by mutual nuclear excitation of the beam particles. The strong enhancement of the production cross section at low transverse momentum is consistent with coherent photoproduction. The π+π−π+π− invariant mass spectrum of the coherent events exhibits a broad peak around 1540±40 MeV/c2 with a width of 570±60 MeV/c2, in agreement with the photoproduction data for the ρ0(1700). We do not observe a corresponding peak in the π+π− final state and measure an upper limit for the ratio of the branching fractions of the ρ0(1700) to π+π− and π+π−π+π− of 2.5% at 90% confidence level. The ratio of ρ0(1700) and ρ0(770) coherent production cross sections is measured to be 13.4±0.8stat.±4.4syst.%.Received 4 December 2009DOI:https://doi.org/10.1103/PhysRevC.81.044901©2010 American Physical Society