Ratios of collective flow observables in high-energy isobar collisions are insensitive to final-state interactions
C. Zhang, S. Bhatta, J. Jia
Abstract
The ratios of bulk observables, such as harmonic flow ${v}_{2}$ and ${v}_{3}$, between high-energy $^{96}\mathrm{Ru}+^{96}\mathrm{Ru}$ and $^{96}\mathrm{Zr}+^{96}\mathrm{Zr}$ collisions were recently argued to be a clean probe of the nuclear structure differences between $^{96}\mathrm{Ru}$ and $^{96}\mathrm{Zr}$. Using a transport model simulation of isobar collisions, we quantify this claim from the dependence of the ratios ${v}_{2,\mathrm{Ru}}/{v}_{2,\mathrm{Zr}}$ and ${v}_{3,\mathrm{Ru}}/{v}_{3,\mathrm{Zr}}$ on various final-state effects, such as the shear viscosity, hadronization, and hadronic cascade. Although the ${v}_{2}$ and ${v}_{3}$ change by more than 50% when varying the final-state effects, the ratios are unchanged. In addition, these ratios are independent of the transverse momentum ${p}_{\mathrm{T}}$ and hadron species, despite of up to a factor of 2 change in ${v}_{n}$. The ratio of mean transverse momentum $\ensuremath{\langle}{p}_{\mathrm{T}}\ensuremath{\rangle}$ is found to be controlled by the nuclear skin and nuclear radius but is only slightly impacted by the final-state effects. Therefore, these isobar ratios serve as a clean probe of the initial condition of the quark-gluon plasma, which, in turn, is controlled by the collective structure of the colliding nuclei.