Observation of strong nuclear suppression in exclusive $J/\psi$ photoproduction in Au$+$Au ultra-peripheral collisions at RHIC

TL;DR

This paper reports the first measurement of the correlation between $J/psi$ photoproduction and neutron emission in ultra-peripheral Au+Au collisions at RHIC, revealing nuclear suppression effects in both coherent and incoherent processes.

Contribution

It introduces the first experimental measurement of the rapidity correlation between $J/psi$ production and nuclear breakups, and quantifies nuclear suppression factors for the first time.

Findings

Coherent $J/psi$ cross section is 71% of free proton value.

Incoherent $J/psi$ cross section is 36% of free proton value.

Stronger suppression in incoherent production offers new insights into initial-state parton densities.

Abstract

We report a measurement of exclusive $J/\psi$ photoproduction in Au$+$Au ultra-peripheral collisions at $\sqrt{s_{_\mathrm{NN}}}=200$ GeV using the STAR detector. For the first time, i) the rapidity correlation between $J/\psi$ photoproduction and neutron emission from nuclear breakups has been experimentally measured; ii) nuclear suppression factors are measured for both the coherent and incoherent $J/\psi$ production. At photon-nucleon center-of-mass energy of 25.0 GeV, the coherent and incoherent $J/\psi$ cross sections of Au nuclei are found to be $71\pm10\%$ and $36\pm7\%$, respectively, of that of free protons. The stronger suppression observed in the incoherent production provides a new experimental handle to study the initial-state parton density in heavy nuclei. Data are compared with theoretical models quantitatively.