Influence of the neutron-skin effect on nuclear isobar collisions at RHIC

Abstract

The unambiguous observation of a Chiral Magnetic Effect (CME)-driven charge separation is the core aim of the isobar program at RHIC consisting of ${}_{40}^{96}$Zr+${}_{40}^{96}$Zr and ${}_{44}^{96}$Ru+${}_{44}^{96}$Ru collisions at $\sqrt{s_{\mathrm{NN}}}=200$ GeV. We quantify the role of the spatial distributions of the nucleons in the isobars on both eccentricity and magnetic field strength within a relativistic hadronic transport approach (SMASH, Simulating Many Accelerated Strongly-interacting Hadrons). In particular, we introduce isospin-dependent nucleon-nucleon spatial correlations in the geometric description of both nuclei, deformation for ${}_{44}^{96}$Ru and the so-called neutron skin effect for the neutron-rich isobar i.e. ${}_{40}^{96}$Zr. The main result of this study is a reduction of the magnetic field strength difference between ${}_{44}^{96}$Ru+${}_{44}^{96}$Ru and ${}_{40}^{96}$Zr+${}_{40}^{96}$Zr by a factor of 2, from $10\%$ to $5\%$ in peripheral collisions when the neutron-skin effect is included. Further, we find an increase of eccentricity by up to 10$\%$ when deformation is taken into account while neither the neutron skin effect nor the nucleon-nucleon correlations result into a significant modification of this observable with respect to the traditional Woods-Saxon modeling. Our results suggest a significantly smaller CME signal to background ratio for the experimental charge separation measurement in peripheral collisions with the isobar systems than previously expected.

Comments: 6 pages, 3 figures, v2: bug in the implementation of the deformation has been fixed. Conclusions remain intact