2026/07/21 by Hadi Mehrabpour, Zahra Sheibani, Li Yan +2
#nucl-th #hep-ex #hep-ph #nucl-ex
Determining the role of intrinsic hexadecapole deformation (β4) in nuclear structure remains a long-standing challenge. Relativistic heavy-ion collisions provide a unique opportunity to address this problem by converting the initial nuclear geometry into the collective motion of the quark--gluon plasma (QGP). Using event-by-event viscous hydrodynamic simulations of ultra-central 238U+238U collisions at √sNN=193 GeV, we investigate whether higher-order collective flow can isolate the contribution of β4 and test the β2-β4 correlation. We demonstrate that information carried by the sign of β4 survives the QGP evolution and is enhanced through nonlinear hydrodynamic response: the fourth-order flow harmonic acquires its topology dependence predominantly from the linear response, whereas the sensitivity of the sixth-order harmonic originates almost entirely from nonlinear mode coupling. As a consequence, the nonlinear response coefficient ξ6,222 cleanly separates the (β2,β4) intrinsic nuclear topologies. These results establish the sign of β4 as an experimentally accessible signature of deviations from the quadrupole--hexadecapole correlation, demonstrating that higher-order collective flow provides a direct probe of nuclear multipole structure while revealing how nonlinear QGP dynamics encode subtle higher-order geometric information into final-state observables.