Possible octupole deformation of Pb 208 and the ultracentral v2 to v3 puzzle

Recent measurements have established the sensitivity of ultracentral heavy-ion collisions to the deformation parameters of nonspherical nuclei. In the case of Xe129 collisions, a quadrupole deformation of the nuclear profile led to an enhancement of elliptic flow in the most central collisions. In Pb208 collisions a discrepancy exists in similar centralities, where either elliptic flow is overpredicted or triangular flow is underpredicted by hydrodynamic models; this is known as the v2-to-v3 puzzle in ultracentral collisions. Motivated by low-energy nuclear structure calculations, we consider the possibility that Pb208 nuclei could have a pear-shape deformation (octupole), which has the effect of increasing triangular flow in central PbPb collisions. Using the recent data from ALICE and ATLAS, we reexamine the v2-to-v3 puzzle in ultracentral collisions, including new constraints from recent measurements of the triangular cumulant ratio v34/v32 and comparing two different hydrodynamic models. We find that while an octupole deformation would slightly improve the ratio between v2 and v3, it is at the expense of a significantly worse triangular flow cumulant ratio. In fact, the latter observable prefers no octupole deformation, with β3 0.0375 for Pb208, and is therefore consistent with the expectation for a doubly-magic nucleus even at top collider energies. The v2-to-v3 puzzle remains a challenge for hydrodynamic models.