Speaker
Description
Relativistic heavy-ion collisions offer a unique opportunity to probe the deformation of colliding nuclei. The quadrupole deformation parameter $\beta_{2}$ has been extensively studied through the flow coefficient $v_2$ in such collisions, as well as through low-energy measurements of the electric quadrupole transition probabilities, $\rm B(E_{2})$. In contrast, the hexadecapole deformation parameter ($\beta_{4}$) has often been neglected in modeling heavy-ion collisions, primarily due to large experimental and theoretical uncertainties. The non-linear response coefficient $\chi_{4,22}$ quantifies the strength of the coupling between the second-order and the fourth-order flow harmonic. This coefficient is sensitive to higher-order nuclear deformations such as $\beta_{4}$, which modulate the initial geometry beyond simple ellipticity. In this talk, we present measurements of $\chi_{4,22}$ in Au+Au and U+U collisions at 200 GeV and 193 GeV, respectively. A deviation of the $\chi_{4,22}$ ratio between U+U and Au+Au collisions from unity was observed in central collisions. The results were compared with calculations from the iEBE-VISHNU hydrodynamic and AMPT models, where the $^{238}$U $\beta_4$ predicted by nuclear structure theory calculations is incorporated. The comparison indicates that the observed deviation favors a sizeable hexadecapole deformation of the $^{238}$U nucleus.
