Search for the chiral magnetic effect in collisions between two isobars with deformed and neutron-rich nuclear structures

Abstract

Isobar collisions, which were thought to have the same background and different magnetic fields, provide an opportunity to verify the chiral magnetic effect (CME) in relativistic heavy-ion collisions. However, the first result from the RHIC-STAR isobar experiment did not observe the CME signal, but discovered that the backgrounds are different between $_{44}^{96}\mathrm{Ru}+_{44}^{96}\mathrm{Ru}$ and $_{40}^{96}\mathrm{Zr}+_{40}^{96}\mathrm{Zr}$ collisions. We test eighteen cases of Woods-Saxon parameter settings resulting from different nuclear deformation or nuclear structure effects using a mutiphase transport model. We find out that seven cases can reasonably reproduce three reference ratios measured by the STAR experiment. Considering both the halo-type neutron skin structure and CME-like charge separation, we demonstrate that it is difficult for the CME observables ($\mathrm{\ensuremath{\Delta}}\ensuremath{\delta}, \mathrm{\ensuremath{\Delta}}\ensuremath{\delta}$ ratio, $\mathrm{\ensuremath{\Delta}}\ensuremath{\gamma}$, and $\mathrm{\ensuremath{\Delta}}\ensuremath{\gamma}$ ratio) to distinguish the presence or absence of the CME, if the CME strength is weak in isobar collisions. It is because the final state interactions significantly weaken the initial CME signal, resulting in nonlinear sensitivities of the CME observables to the CME strength. Therefore, more sensitive observables are required to search for the possible small CME signal in isobar collisions.