Photoproduction in Ultra-Peripheral Relativistic Heavy Ion Collisions with STAR

TL;DR

This paper reports on recent measurements of photoproduction processes in ultra-peripheral heavy ion collisions at STAR, including cross sections, helicity analysis, and comparisons with theoretical models, advancing understanding of electromagnetic interactions in nuclear physics.

Contribution

It provides new experimental data on $ ho^0$ production, nuclear excitation, and 4-pion states in ultra-peripheral collisions, with detailed analysis and comparison to theoretical predictions.

Findings

Measured $ ho^0$ production cross sections for coherent and incoherent processes.

Found $ ho^0$ helicity matrix elements consistent with s-channel helicity conservation.

Observed the 4-pion final state in ultra-peripheral collisions.

Abstract

We present a summary of recent photoproduction results in ultra peripheral relativistic heavy ions collisions with STAR. These collisions have impact parameters larger then twice the nuclear radius; the nuclei do not physically collide, but interact via long-range electromagnetic fields. We observe exclusive $\rho^0$ production as well as $AuAu\to Au^*Au^* \rho^0$ with accompanying mutual nuclear excitation at $\sqrt{s_{NN}}=200$ GeV. We report the $\rho^0$ production cross section for both coherent and incoherent coupling accompanied by mutual nuclear excitation. We have studied the cross section as a function of $p_T$, $y_{\rho^0}$ and $M_{\pi\pi}$ and compared it to theoretical models. In addition, we measured the $\rho^0$ helicity matrix elements. They are found to be consistent with s-channel helicity conservation. The ratio of coherent $\rho^0$ and direct $\pi^+\pi^-$ pair photoproduction has been measured and found to be consistent with earlier measurements. The 4-pion final state $AuAu \to \pi^+\pi^-\pi^+\pi^-$ state has also been observed.