The Matter Radius of 132Sn and the CREX-PREX Dilemma

The density dependence of the nuclear symmetry energy remains a central open problem in nuclear physics. Parity violating electron scattering experiments have provided largely model-independent determinations of the neutron skin thickness of both 48Ca and 208Pb, whose consistent theoretical interpretation remains challenging. A new measurement of the matter radius of the unstable, doubly magic nucleus 132Sn provides an important additional constraint. Using a representative set of covariant energy density functionals spanning a wide range of isovector properties, we show that at least one of these models can simultaneously reproduce the charge and matter radii of132Sn. When interpreted together with the PREX and CREX results, the new measurement--much like CREX--favors a relatively soft symmetry energy. These findings underscore the need for an independent confirmation of the PREX result, such as the anticipated MREX campaign at the MESA facility.