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

Abstract

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.