The Mainz Radius EXperiment (MREX) is all about measuring the neutron skin of heavy nuclei using Parity-violating electron scattering.
The basic properties of neutron stars such as their mass, radius, and cooling behaviour are determined by the nuclear Equation of State (EOS). A key unknown of the EOS is the density dependence of the symmetry energy, which quantifies the change in nuclear energy associated with modifying the neutron-proton asymmetry.
Heavy nuclei, even though orders of magnitude less massive than neutron stars are governed by the same physics, which is enshrined in the EOS.
In particular the neutron radius of a heavy nucleus is strongly correlated to several neutron-star observables. The size of a heavy nucleus is thought to be determined by its neutrons since they are more numerous than its protons. Where do the extra neutrons go? Are they at the center or are some of them at the surface forming a neutron-rich skin?
This simple question involves many important issues. Information about the location of neutrons inside a nucleus has been historically elusive. An accurate determination of the thickness of the neutron skin of a nucleus would provide a unique experimental constraint on the symmetry energy and ultimately on some critical properties of supernovae and neutron stars.
In the Standard Model the weak charge of the neutron is large, whereas it is small for the proton. Parity-violating electron scattering therefore provides a very clean probe of neutron densities.
Description of MREX experiment etc