Chiral Magnetic Effect In Collisions Research Articles

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

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.

Methods for a blind analysis of isobar data collected by the STAR collaboration

In 2018, the STAR collaboration collected data from \(_{44}^{96}{\mathrm{Ru}}+_{44}^{96}{\mathrm{Ru}}\) and \(_{40}^{96}{\mathrm{Zr}}+_{40}^{96}{\mathrm{Zr}}\) at \(\sqrt{s_\text {NN}}=200\) GeV to search for the presence of the chiral magnetic effect in collisions of nuclei. The isobar collision species alternated frequently between \(_{44}^{96}{\mathrm{Ru}}+_{44}^{96}{\mathrm{Ru}}\) and \(_{40}^{96}{\mathrm{Zr}}+_{40}^{96}{\mathrm{Zr}}\). In order to conduct blind analyses of studies related to the chiral magnetic effect in these isobar data, STAR developed a three-step blind analysis procedure. Analysts are initially provided a “reference sample” of data, comprised of a mix of events from the two species, the order of which respects time-dependent changes in run conditions. After tuning analysis codes and performing time-dependent quality assurance on the reference sample, analysts are provided a species-blind sample suitable for calculating efficiencies and corrections for individual \(\approx 30\)-min data-taking runs. For this sample, species-specific information is disguised, but individual output files contain data from a single isobar species. Only run-by-run corrections and code alteration subsequent to these corrections are allowed at this stage. Following these modifications, the “frozen” code is passed over the fully un-blind data, completing the blind analysis. As a check of the feasibility of the blind analysis procedure, analysts completed a “mock data challenge,” analyzing data from \({\mathrm{Au}}+{\mathrm{Au}}\) collisions at \(\sqrt{s_\text {NN}}=27\) GeV, collected in 2018. The \({\mathrm{Au}}+{\mathrm{Au}}\) data were prepared in the same manner intended for the isobar blind data. The details of the blind analysis procedure and results from the mock data challenge are presented.

Impact of magnetic-field fluctuations on measurements of the chiral magnetic effect in collisions of isobaric nuclei

We investigate the properties of electromagnetic fields in isobaric $_{44}^{96}\textrm{Ru}+\,_{44}^{96}\textrm{Ru}$ and $_{40}^{96}\textrm{Zr}+\,_{40}^{96}\textrm{Zr}$ collisions at $\sqrt{s}$ = 200 GeV by using a multiphase transport model, with special emphasis on the correlation between magnetic field direction and participant plane angle $\Psi_{2}$ (or spectator plane angle $\Psi_{2}^{\rm SP}$), i.e. $\langle{\rm cos}\ 2(\Psi_B - \Psi_{2})\rangle$ [or $\langle{\rm cos}\ 2(\Psi_B - \Psi_{2}^{\rm SP})\rangle$]. We confirm that the magnetic fields of $_{44}^{96}\textrm{Ru}+\,_{44}^{96}\textrm{Ru}$ collisions are stronger than those of $_{40}^{96}\textrm{Zr}+\,_{40}^{96}\textrm{Zr}$ collisions due to their larger proton fraction. We find that the deformation of nuclei has a non-negligible effect on $\langle{\rm cos}\ 2(\Psi_B - \Psi_{2})\rangle$ especially in peripheral events. Because the magnetic-field direction is more strongly correlated with $\Psi_{2}^{\rm SP}$ than with $\Psi_{2}$, the relative difference of the chiral magnetic effect observable with respect to $\Psi_{2}^{\rm SP}$ is expected to be able to reflect much cleaner information about the chiral magnetic effect with less influences of deformation.