Constraining URCA cooling of neutron stars from the neutron radius of [Formula Presented]

Recent observations by the Chandra observatory suggest that some neutron stars may cool rapidly, perhaps by the direct URCA process which requires a high proton fraction. The proton fraction is determined by the nuclear symmetry energy whose density dependence may be constrained by measuring the neutron radius of a heavy nucleus, such as [Formula Presented] Such a measurement is necessary for a reliable extrapolation of the proton fraction to the higher densities present in a neutron star. A large neutron radius in [Formula Presented] implies a stiff symmetry energy that grows rapidly with density, thereby favoring a high proton fraction and allowing direct URCA cooling. Predictions for the neutron radius in [Formula Presented] are correlated to the proton fraction in dense matter by using a variety of relativistic effective field-theory models. Models that predict a neutron [Formula Presented] minus proton [Formula Presented] root-mean-square radius in [Formula Presented] to be [Formula Presented] have proton fractions too small to allow the direct URCA cooling of [Formula Presented] neutron stars. Conversely, if [Formula Presented] the direct URCA process is allowed (by all models) to cool down a [Formula Presented] neutron star. The Parity Radius Experiment at Jefferson Laboratory aims to measure the neutron radius in [Formula Presented] accurately and model independently via parity-violating electron scattering. Such a measurement would greatly enhance our ability to either confirm or dismiss the direct URCA cooling of neutron stars.