Global polarization of $\Lambda$ and $\bar{\Lambda }$ hyperons in Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}=19.6$ and $27$ GeV

Abstract

In relativistic heavy-ion collisions, a global spin polarization, $P_{\mathrm{H}}$, of $\Lambda$ and $\bar{\Lambda }$ 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 $\sqrt{s_{\mathrm{NN}}}$. A splitting between $\Lambda$ and $\bar{\Lambda }$ 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 $\sqrt{s_{\mathrm{NN}}}=19.6$ and $27$ GeV in the RHIC Beam Energy Scan Phase II using the STAR detector, with an upper limit of $P_{\bar{\Lambda }}-P_{\Lambda }<0.24$% and $P_{\bar{\Lambda }}-P_{\Lambda }<0.35$%, respectively, at a 95% confidence level. We derive an upper limit on the naïve extraction of the late-stage magnetic field of $B<9.4\cdot 10^{12}$ T and $B<1.4\cdot 10^{13}$ T at $\sqrt{s_{\mathrm{NN}}}=19.6$ 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 $|y|<1$ 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.

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• Dirac fermions under imaginary rotation
• Global $\Lambda$ hyperon polarization in low-energy heavy ion collisions -- a scenario without vorticity
• Dissociation-driven quarkonium spin alignment in Pb--Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV
• Practical Considerations for Measuring Global Spin Density Matrix Elements of Vector Mesons in Heavy-Ion Collisions
• Perfect spinfluid: A divergence-type approach
• Physics of strong electromagnetic fields in relativistic heavy-ion collisions