In relativistic heavy-ion collisions, a global spin polarization, $P_{\mathrm{H}}$, of $\mathrm{\Lambda }$ and $\overline{\mathrm{\Lambda }}$ hyperons along the direction of the system angular momentum was discovered and measured across a broad range of collision energies and demonstrated a trend of increasing $P_{\mathrm{H}}$ with decreasing $\sqrt{s_{NN}}$. A splitting between $\mathrm{\Lambda }$ and $\overline{\mathrm{\Lambda }}$ polarization may be possible due to their different magnetic moments in a late-stage magnetic field sustained by the quark-gluon plasma which is formed in the collision. The results presented in this study find no significant splitting at the collision energies of $\sqrt{s_{NN}}=19.6$ and 27 GeV in the BNL Relativistic Heavy Ion Collisions Beam Energy Scan Phase II using the STAR detector, with an upper limit of $P_{\overline{\mathrm{\Lambda }}}-P_{\mathrm{\Lambda }}<0.24$% and $P_{\overline{\mathrm{\Lambda }}}-P_{\mathrm{\Lambda }}<0.35$%, respectively, at a 95% confidence level. We derive an upper limit on the naive extraction of the late-stage magnetic field of $B<9.4\times {10}^{12}$ T and $B<1.4\times {10}^{13}$ T at $\sqrt{s_{NN}}=19.6$ and 27 GeV, respectively, although more thorough derivations are needed. Differential measurements of $P_{\mathrm{H}}$ were performed with respect to collision centrality, transverse momentum, and rapidity. With our current acceptance of $\left|y\right|<1$ and uncertainties, we observe no dependence on transverse momentum and rapidity in this analysis. These results challenge multiple existing model calculations following a variety of different assumptions which have each predicted a strong dependence on rapidity in this collision-energy range.

• Received 17 May 2023
• Accepted 20 June 2023

DOI:https://doi.org/10.1103/PhysRevC.108.014910