Speaker
Description
In ultra-peripheral collisions (UPCs), coherent J/$\psi$ photoproduction has been recognized as one of the most sensitive probes of the nuclear gluon distribution. Recently, STAR published differential measurements on photoproduced J/$\psi$ in ultra-peripheral d+Au and Au+Au collisions at $\sqrt{\mathrm{s_{NN}}}$ = 200 GeV. These results provide important constraints on gluon distribution functions and sub-nucleonic shape fluctuations in both light and heavy nuclei. Compared to d+Au and Au+Au collisions, the collision system size in isobaric collisions ($^{96}_{44}Ru$ + $^{96}_{44}Ru$ and $^{96}_{40}Zr$ + $^{96}_{40}Zr$) lies in between. Therefore, the measurement of coherent J/$\psi$ photoproduction in isobaric UPCs offers a unique opportunity to study the system size dependence of gluon structure.
In this talk, we present the differential cross sections of photoproduced coherent J/$\psi$ as a function of rapidity (y) in $^{96}_{44}Ru$ ($^{96}_{40}Zr$) + $^{96}_{44}Ru$ ($^{96}_{40}Zr$) UPCs at $\sqrt{\mathrm{s_{NN}}}$ = 200 GeV. The results will also be shown for different combinations of neutron emission, where neutrons are detected by zero degree calorimeters, which help resolve the photon-gluon emitter ambiguity. More importantly, these data provide crucial constraints on the system size dependence of the gluon structure within nuclei in the kinematic range x$_{parton}$, the momentum fraction carried by the gluon, $\sim 0.015-0.03 $. The results are compared with theoretical model calculations and previous STAR measurements, and the physics implications are discussed.
