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 $\langle p_{\mathrm{T}}\rangle$ 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.

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• Collective flow
• Quark-gluon plasma
• Relativistic heavy-ion collisions