Characterizing the Neutron Skin of $^{48}$Ca Through Collective Flow at the CERN Large Hadron Collider

Abstract: The recently developed ``imaging-by-smashing" technique has emerged as a powerful approach to connect final-state collective flow phenomena in ultra-relativistic nuclear collisions with the intrinsic structure of the colliding nuclei. While most efforts have focused on constraining nuclear shape properties such as deformation and triaxiality, less attention has been given to the neutron skin, primarily in heavy nuclei such as $^{208}$Pb. In this work, a novel study of the neutron-skin thickness in $^{48}$Ca is presented, based on comparative analyses of $^{48}$Ca+$^{48}$Ca and $^{40}$Ca+$^{40}$Ca collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV. Simulations within the AMPT framework are employed to investigate the impact of varying neutron-skin thicknesses $Δr_{np}$ for $^{48}$Ca on collective flow observables, including anisotropic flow coefficients and mean transverse momentum ($[p_{\rm T}]$) fluctuations. The triangular flow $v_3$, quadrangular flow $v_4$, as well as the variance and skewness of $[p_{\rm T}]$ fluctuations, display notable sensitivity to $Δr_{np}$. These findings indicate that calcium-isotope runs at the LHC could provide an independent and complementary approach to constraining $Δr_{np}$, with the potential to help resolve the current tension between PREX and CREX measurements.