Title: Probing the gluonic structure of the deuteron with $J/\psi$ photoproduction in d+Au ultra-peripheral collisions

Zhoudunming Tu, for the STAR Collaboration

Brookhaven National Laboratory, Upton, NY 11973

Nov 1, 2021

Abstract:

Diffractive vector meson production has been considered one of the most sensitive experimental probes to the inner structure of nucleon and nucleus. In recent years, photoproduction of $J/\psi$ in ultra-peripheral collisions (UPCs) of heavy ions at the Large Hadron Collider (LHC) and Relativistic Heavy-Ion Collider (RHIC) have provided valuable data at low Bjorken $x$ regime (e.g., $x<0.01$), which is mostly sensitive to the gluon structure.  In heavy nucleus, e.g., lead or gold, the gluon distributions are found to be significantly modified relative to a free proton, while the origin of this modification remains unknown. In this talk, we present the first experimental measurement of $J/\psi$ photoproduction off the deuteron in UPCs at $\sqrt{_{_NN}}=200$ GeV in d+Au collisions. The differential cross section as a function of momentum transfer −t is measured. In addition, data with a neutron tagged in the deuteron-going Zero-Degree Calorimeter is investigated for the first time, which is found to be consistent with the expectation of incoherent diffractive scattering at low momentum transfer. Theoretical predictions based on the Color Glass Condensate saturation model and the gluon shadowing model are compared with the data quantitatively. The reported measurement are found to be directly sensitive to the gluon density distribution of the deuteron and the deuteron breakup, which provides insights into the nuclear gluonic structure.