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

Abstract

The STAR isobar data of ${}^{96}$Ru+${}^{96}$Ru and ${}^{96}$Zr+${}^{96}$Zr collisions at $\sqrt{s_{\mathrm{NN}}}=200$ GeV show that ratios of observables ($R_{\mathcal{O}}$) such as the multiplicity distribution, $p(N_{\mathrm{ch}})$, and the harmonic flow, $v_n$, deviate from unity, when presented as a function of centrality, $c$. These deviations have been attributed to the differences in the shape and radial profiles between ${}^{96}$Ru and ${}^{96}$Zr nuclei. In addition, the ratios $R_{\mathcal{O}}(x)$ depend on the choice of the event activity variable $x$, which could be either $N_{\mathrm{ch}}$ or centrality. We estimate the difference $\Delta R$ between these two choices, based on the published $p(N_{\mathrm{ch}})$, 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 $R_{v_n}(c)$, $R_{v_n}(N_{\mathrm{ch}})$ is nearly independent of the analysis approaches, suggesting that nonflow effects are better controlled by $N_{\mathrm{ch}}$ than $c$. The ratios of observables sensitive to the chiral magnetic effect (CME) are also much closer to unity for $x=N_{\mathrm{ch}}$ than $x=c$, indicating that the ratios calculated at the same $N_{\mathrm{ch}}$ 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 $p(N_{\mathrm{ch}})$ is also used to estimate $\Delta R_{⟨p_{\mathrm{T}}⟩}$ for mean transverse momentum, which is non-negligible compared with $R_{⟨p_{\mathrm{T}}⟩}-1$.

6 pages, 6 figures, published version

References in corpus (8)

• The Chiral Magnetic Effect
• Testing the Chiral Magnetic Effect with Central U+U collisions
• Event-by-event mean $p_{\mathrm{T}}$ fluctuations in pp and Pb-Pb collisions at the LHC
• Probing triaxial deformation of atomic nuclei in high-energy heavy ion collisions
• Determine the neutron skin type by relativistic isobaric collisions
• Evidence of the triaxial structure of ${}^{129}$Xe at the Large Hadron Collider
• Probing nuclear quadrupole deformation from correlation of elliptic flow and transverse momentum in heavy ion collisions
• Two- and three-particle nonflow contributions to the chiral magnetic effect measurement by spectator and participant planes in relativistic heavy ion collisions

Cited by in corpus (7)

• Separating the impact of nuclear skin and nuclear deformation in high-energy isobar collisions
• Scaling approach to nuclear structure in high-energy heavy-ion collisions
• Ratios of collective flow observables in high-energy isobar collisions are insensitive to final state interactions
• Search for the chiral magnetic effect in collisions between two isobars with deformed and neutron-rich nuclear structures
• Examination of nucleon distribution with Bayesian imaging for isobar collisions
• Momentum dependent flow correlations in deformed nuclei at collision energies available at the BNL Relativistic Heavy Ion Collider
• Exploring Rapidity-Even Dipolar Flow in Isobaric Collisions at RHIC