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Sep 24 – 29, 2023
US/Eastern timezone

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

Sep 26, 2023, 10:10 AM
20m
Junior A3 (Durham Convention Center)

Junior A3

Durham Convention Center

Talk Spin in Heavy Ion Collisions Spin in Heavy Ion Collisions

Speaker

Dandan Shen

Description

The large scale set by the charm mass means the heavy quark pairs are produced early in heavy-ion collisions and experience the full evolution of the quark-gluon plasma (QGP) created in these collisions. Because of this, $J/\psi$ serves as one of the important probes to study the properties of the QGP. In Ru+Ru and Zr+Zr collisions 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 the $J/\psi$'s strong coupling with the medium and its potentially small regeneration contribution, respectively. Besides those observable, the $J/\psi$ polarization can shed new light on the QGP properties and the $J/\psi$ production mechanism in heavy-ion collisions. For example, it has been hypothesized that the $J/\psi$ polarization can be observed due to the spin-orbit coupling between $J/\psi$ and the QGP's large angular momentum in non-central heavy-ion collisions. The early production of $J/\psi$ also makes its polarization potentially sensitive to the strong magnetic field at the early stage.
In this talk, we will 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 in the mid-rapidity ($|y| < 1$) and the $J/\psi$ transverse momentum range of $0.2 < p_T < 10$ GeV/$c$. The $J/\psi$ polarization parameters are measured in the Helicity frame, Collins-Soper frame and with respect to the event plane. We conclude by presenting the physics implications of this measurement.

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