Evidence of Quadrupole and Octupole Deformations in ^{96}Zr+^{96}Zr and ^{96}Ru+^{96}Ru Collisions at Ultrarelativistic Energies.

Abstract

In the hydrodynamic model description of heavy-ion collisions, the elliptic flow v_{2} and triangular flow v_{3} are sensitive to the quadrupole deformation β_{2} and octupole deformation β_{3} of the colliding nuclei. Therelations between v_{n} and β_{n} have recently been clarified and were found to follow a simple parametric form. The STAR Collaboration has just published precision v_{n} data from isobaric ^{96}Ru+^{96}Ru and ^{96}Zr+^{96}Zr collisions, where they observe large differences in central collisions v_{2,Ru}>v_{2,Zr} and v_{3,Ru}<v_{3,Zr}. Using a transport model simulation, we show that these orderings are a natural consequence of β_{2,Ru}≫β_{2,Zr} and β_{3,Ru}≪β_{3,Zr}. We reproduce the centrality dependence of the v_{2} ratio qualitatively and v_{3} ratio quantitatively and extract values of β_{2} and β_{3} that are consistent with those measured at low-energy nuclear structure experiments. STAR data provide the first direct evidence of strong octupole correlations in the ground state of ^{96}Zr in heavy-ion collisions. Our analysis demonstrates that flow measurements in high-energy, heavy-ion collisions, especially using isobaric systems, are a new precision tool to study nuclear structure physics.