Global polarization of Λ and Λ¯ hyperons in Au+Au collisions at sNN=19.6 and 27 GeV

Le résumé fourni par la source

In relativistic heavy-ion collisions, a global spin polarization, ${P}_{\mathrm{H}}$, of $\mathrm{\ensuremath{\Lambda}}$ and $\overline{\mathrm{\ensuremath{\Lambda}}}$ 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 $\mathrm{\ensuremath{\Lambda}}$ and $\overline{\mathrm{\ensuremath{\Lambda}}}$ 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 ${P}_{\overline{\mathrm{\ensuremath{\Lambda}}}}\ensuremath{-}{P}_{\mathrm{\ensuremath{\Lambda}}}<0.24$% and ${P}_{\overline{\mathrm{\ensuremath{\Lambda}}}}\ensuremath{-}{P}_{\mathrm{\ensuremath{\Lambda}}}<0.35$%, respectively, at a 95% confidence level. We derive an upper limit on the naive extraction of the late-stage magnetic field of $B<9.4\ifmmode\times\else\texttimes\fi{}{10}^{12}$ T and $B<1.4\ifmmode\times\else\texttimes\fi{}{10}^{13}$ 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 $|y|<1$ 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.

Ce résumé expose les affirmations des auteurs. BNTIC ne l’interprète pas comme une validation indépendante des résultats.