Neutron skin thickness in the droplet model with surface width dependence: Indications of softness of the nuclear symmetry energy

Abstract

We analyze the neutron skin thickness in finite nuclei with the droplet model and effective nuclear interactions. The ratio of the bulk symmetry energy $J$ to the so-called surface stiffness coefficient $Q$ has in the droplet model a prominent role in driving the size of neutron skins. We present a correlation between the density derivative of the nuclear symmetry energy at saturation and the $J/Q$ ratio. We emphasize the role of the surface widths of the neutron and proton density profiles in the calculation of the neutron skin thickness when one uses realistic mean-field effective interactions. Next, taking as experimental baseline the neutron skin sizes measured in 26 antiprotonic atoms along the mass table, we explore constraints arising from neutron skins on the value of the $J/Q$ ratio. The results favor a relatively soft symmetry energy at subsaturation densities. Our predictions are compared with the recent constraints derived from other experimental observables. Though the various extractions predict different ranges of values, one finds a narrow window $L~45\text{\ensuremath{-}}75$ MeV for the coefficient $L$ that characterizes the density derivative of the symmetry energy that is compatible with all the different empirical indications.