Measurement of $J/\psi$ polarization and spin alignment in Ru+Ru and Zr+Zr collisions at $\sqrt{s_{_{\rm NN}}} = 200$ GeV at STAR

The heavy quark pairs are produced early in heavy-ion collisions and experience the full evolution of the Quark-Gluon Plasma created in these collisions. $J/\psi$ serves as one of the important probes to study the properties of the QGP. Using the high-statistics Ru+Ru and Zr+Zr collision data at $\sqrt{s_{_{\rm NN}}} = 200$ GeV recorded by the STAR experiment, it has been observed that the $J/\psi$ yield is strongly suppressed and its elliptic flow is consistent with zero indicating color screening of the heavy quark-antiquark pair potential in the medium and its potentially small regeneration contribution, respectively. Besides those observables, the $J/\psi$ polarization can shed new light on the QGP properties and the $J/\psi$ production mechanism in heavy-ion collisions. In these proceedings, we present the first measurement of $J/\psi$ polarization in heavy-ion collisions at RHIC. The study is carried out by reconstructing the $J/\psi$ through its di-electron decay channel at mid-rapidity ($|y| < 1$). The $J/\psi$ polarization parameters are measured in the Helicity frame, and Collins-Soper frame, and the spin alignment is extracted with respect to the event plane. We conclude by presenting the physics implications of this measurement.