Speaker
Description
The thickness of the neutron skin ($\Delta r_{np}$) in $^{48}$Ca remains uncertain, with parity‑violating electron scattering (CREX) and ab initio calculations favoring a thin skin while energy density functionals prefer larger values. Heavy‑ion collisions at relativistic energies can probe the skin through its geometric imprint on the quark‑gluon plasma, but a thicker skin also increases the effective nuclear matter radius, creating a size–skin degeneracy that complicates interpretation. We resolve this by systematically comparing $^{48}$Ca+$^{48}$Ca and $^{48}$Ti+$^{48}$Ti collisions at $\sqrt{s_{NN}}=5.02$ TeV using the AMPT model with string melting. Three neutron‑skin configurations are implemented for $^{48}$Ca with their limits defined by the CREX upper and lower bound and a zero-skin ($\Delta r_{np}=0$) baseline, together with a size-controlled $^{48}$Ca reference with a matched proton radius that isolates the skin effect from the size effect. We find that the neutron skin is a purely geometric perturbation: the hydrodynamic response coefficients are unchanged, and all flow modifications are inherited from the initial geometry. First‑moment observables exhibit only modest sensitivity to $\Delta r_{np}$, while second‑moment quantities such as variances, linear response correlators, event‑plane alignment, and correlation between shape and size, are substantially amplified. The covariance between elliptic flow and mean transverse momentum fluctuations emerges as the single most sensitive probe, enhanced by up to 25% in the initial state and 12% in the final state for the CREX upper bound. Our results establish that isobaric $^{48}$Ca/$^{48}$Ti collisions at the LHC, combined with multi-observable analysis targeting second-moment and geometry-flow correlators, provide a clean and high-precision pathway to constrain the neutron skin of $^{48}$Ca independently of nuclear scattering experiments.
