Ratios of collective flow observables in high-energy isobar collisions are insensitive to final-state interactions

The ratios of bulk observables, such as harmonic flow v2 and v3, between high-energy Ru96+Ru96 and Zr96+Zr96 collisions were recently argued to be a clean probe of the nuclear structure differences between Ru96 and Zr96. Using a transport model simulation of isobar collisions, we quantify this claim from the dependence of the ratios v2,Ru/v2,Zr and v3,Ru/v3,Zr on various final-state effects, such as the shear viscosity, hadronization, and hadronic cascade. Although the v2 and v3 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 pT and hadron species, despite of up to a factor of 2 change in vn. The ratio of mean transverse momentum (pT) 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.