Holography of Low-Centrality Heavy Ion Collisions

Abstract

Large vorticities in the Quark-Gluon Plasma produced in peripheral collisions studied by the STAR collaboration at the RHIC facility have been deduced from observations of polarizations of $\Lambda$ and $\overline{\Lambda}$ hyperons. Recently, the STAR collaboration has reported on the dependence of these polarizations on centrality, at impact energy 200 GeV and relatively large centralities $\mathcal{C} \geq 20\%$. The polarizations increase slowly with centrality, up to perhaps $\mathcal{C} = \,60 - 70\%$. Here we use a holographic model of the vortical QGP to make predictions regarding these polarizations for smaller centralities, ranging from $5 - 20\%$. The model predicts that as one moves downwards from $20\%$, $\Lambda/\overline{\Lambda}$ polarizations at first decrease but then \emph{increase} sharply, in a characteristic pattern which should be readily detected if collisions can be studied at impact energies below 200 GeV and centrality as low as $5 - 10\%$. The effect should be most evident at moderate impact energies below 200 GeV, so we give predictions for impact energy 27 GeV.