rho^0 Photoproduction in Ultra-Peripheral Relativistic Heavy Ion Collisions with STAR

TL;DR

This paper reports measurements of rho^0 and direct pi^+pi^- photoproduction in ultra-peripheral heavy ion collisions at 200 GeV, comparing experimental results with theoretical models and analyzing the production mechanisms and spin properties.

Contribution

The study provides the first detailed measurement of rho^0 photoproduction in ultra-peripheral heavy ion collisions at RHIC energies, including cross-sections and polarization analysis, and compares results with theoretical predictions.

Findings

Measured coherent cross-section of 530 mb with uncertainties.

Found ratio of incoherent to coherent production to be 0.29.

Observed spin helicity matrix elements consistent with s-channel helicity conservation.

Abstract

Photoproduction reactions occur when the electromagnetic field of a relativistic heavy ion interacts with another heavy ion. The STAR collaboration presents a measurement of rho^0 and direct pi^+pi^- photoproduction in ultra-peripheral relativistic heavy ion collisions at sqrt(s_{NN})=200 GeV. We observe both exclusive photoproduction and photoproduction accompanied by mutual Coulomb excitation. We find a coherent cross-section of sigma(AuAu) -> Au^*Au^*rho^0 = 530 pm 19 (stat.) pm 57 (syst.) mb, in accord with theoretical calculations based on a Glauber approach, but considerably below the predictions of a color dipole model. The rho^0 transverse momentum spectrum (p_{T}^2) is fit by a double exponential curve including both coherent and incoherent coupling to the target nucleus; we find sigma_{inc}/sigma_{coh} = 0.29 pm 0.03 (stat.) pm 0.08 (syst.). The ratio of direct pi^+pi^- to rho^0 production is comparable to that observed in gamma p collisions at HERA, and appears to be independent of photon energy. Finally, the measured rho^0 spin helicity matrix elements agree within errors with the expected s-channel helicity conservation.