Evidence of Quadrupole and Octupole Deformations in Zr 96 + Zr 96 and Ru 96 + Ru 96 Collisions at Ultrarelativistic Energies

In the hydrodynamic model description of heavy-ion collisions, the elliptic flow v2 and triangular flow v3 are sensitive to the quadrupole deformation β2 and octupole deformation β3 of the colliding nuclei. The relations between vn and βn have recently been clarified and were found to follow a simple parametric form. The STAR Collaboration has just published precision vn data from isobaric Ru96+Ru96 and Zr96+Zr96 collisions, where they observe large differences in central collisions v2,Ru>v2,Zr and v3,Ru<v3,Zr. Using a transport model simulation, we show that these orderings are a natural consequence of β2,Ru≫β2,Zr and β3,Ru β3,Zr. We reproduce the centrality dependence of the v2 ratio qualitatively and v3 ratio quantitatively and extract values of β2 and β3 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 Zr96 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.