Speaker
Description
In ultra-peripheral collisions (UPCs), coherent J/$\psi$ photoproduction provides a sensitive probe of the nuclear gluon distributions. The STAR collaboration has reported differential measurements of photoproduced J/$\psi$ in Au+Au UPCs, providing important constraints on nuclear gluon structure and sub-nucleonic fluctuations in heavy nuclei. Compared with Au+Au collisions, isobaric systems ($^{96}_{44}$Ru+$^{96}_{44}$Ru and $^{96}_{40}$Zr+$^{96}_{40}$Zr) feature smaller nuclear sizes and different nuclear structures. Measurements in these collision systems provide a unique opportunity to investigate the system-size dependence of coherent J/$\psi$ photoproduction and nuclear modifications of the gluon distribution.
In this talk, the first cross section measurements of coherent $J/\psi$ photoproduction in ultra-peripheral Ru+Ru and Zr+Zr collisions at $200$ GeV will be presented. Differential cross sections are measured as functions of rapidity and forward neutron multiplicity. A significant suppression of coherent $J/\psi$ photoproduction relative to the Impulse Approximation is observed in both collision systems, indicating strong nuclear shadowing effects. By correcting for the photon flux, photon--nucleus cross sections are extracted at an average photon-nucleon center-of-mass energy of $25$ GeV. Together with previous Au+Au measurements, the extracted cross sections are found to follow a power-law dependence on the nuclear mass number ($A$): $\sigma(\gamma A \rightarrow J/\psi A)\propto A^{\alpha}$ with $\alpha = 0.95 \pm 0.08$. These measurements provide new constraints on nuclear gluon distribution and the system-size dependence of nuclear shadowing effects.
