Impact of event activity variable on the ratio observables in isobar collisions

The STAR isobar data of Ru-96+Ru-96 and Zr-96+Zr-96 collisions at root s(NN) = 200GeV show that ratios of observables (R-O) such as the multiplicity distribution, p(N-ch), and the harmonic flow, v(n), deviate from unity, when presented as a function of centrality, c[1]. These deviations have been attributed to the differences in the shape and radial profiles between Ru-96 and Zr-96 nuclei. In addition, the ratios R-O(x) depend on the choice of the event activity variable x, which could be either N-ch or centrality. We estimate the difference Delta(R) between these two choices, based on the published p(N-ch), as well as those from a multiphase transport (AMPT) model with varied nuclear structure parameters: nuclear radius (R-0), surface diffuseness (a(0)), quadrupole deformation (beta(2)), and octupole deformation (beta(3)). In contrary to R-vn(c), R-vn (N-ch) is nearly independent of the analysis approaches, suggesting that nonflow effects are better controlled by N-ch than c. The ratios of observables sensitive to the chiral magnetic effect (CME) are also much closer to unity for x = N-ch than x = c, indicating that the ratios calculated at the same N-ch provide a better baseline for the non-CME background. According to the AMPT results, the dominant parameter for Delta(R) is a(0), while R-0 and beta(n) are only important in central collisions. The published p(N-ch) is also used to estimate Delta R ( pT ) for mean transverse momentum, which is non-negligible compared with R- pT - 1. (C) 2022 The Author(s). Published by Elsevier B.V.