Session B05: From Neutron Skins to Neutron Stars

Illuminating nuclei for neutron stars

The coming decades are set to provide a much deeper understanding of nuclei and neutron stars. The recent observation of neutron star mergers with gravitational and electromagnetic messengers opens a new window to investigate hadronic matter at high densities. The atomic nucleus provides a mini laboratory to extract physical quantities relevant to our understanding of neutron star physics, such as the equation of state for neutron rich matter and also the role of hadronic degrees of freedom beyond the nucleon. Experiments on atomic nuclei at the latest generation of electromagnetic beam facilities, such as the Jefferson Laboratory (USA) and MAMI (Germany), can address long standing issues in the field. The spatial distribution of neutrons in nuclei is known to a much poorer accuracy than the corresponding proton distribution. However, the difference in these distributions (the “neutron skin” when expressed as a difference in rms radii) constrains the density dependence of the symmetry energy, a parameter which also has a significant role in constraining neutron star structure, cooling and merger physics. In the talk I will outline the current status and future plans in our programme of neutron skin measurements with the Crystal Ball at MAMI. The intense polarized electromagnetic beams, large acceptance detector systems and nucleon spin polarimeters available at modern facilities also offer the opportunity to investigate the role of non-nucleonic degrees of freedom. These may emerge at the higher densities involved in the cores of heavy neutron stars and the high matter densities experienced in mergers. Our recent results obtained at MAMI and Jefferson Lab will be presented along with plans for the future.   

Measurements of Neutron Skins through Parity-Violating Electron Scattering

The neutron densities in atomic nuclei are notoriously difficult to observe with high precision: the standard tool of electromagnetic interactions which has been used to map out the nuclear charge distributions is simply insensitive to neutrons. However, encoded in the small differences between the proton and neutron densities is a wealth of important information about the equation of state for asymmetric nuclear systems and the density dependence of the symmetry energy. Such information is crucial not only in modeling nuclei, but also extreme systems such as neutron stars and with the advent of gravitational wave astronomy there are exciting new opportunities to test such models. Fortunately, nature provides a novel way to image this side of the nucleus: through fundamental weak force interactions, which interact primarily to neutrons rather than protons, in contrast to the electromagnetic interaction.. Presented will be why these neutron distributions play an important part in our understanding of nuclear physics, how one images this aspect of such systems with electron beams, and the recent and upcoming experimental efforts for such measurements.   

Neutron skins and neutron stars in the multi-messenger era

The historical first detection of a binary neutron star merger by the LIGO-Virgo collaboration is providing fundamental new insights into the astrophysical site for the r-process and on the nature of dense matter. Limits on the tidal polarizability inferred from the gravitational wave signal translate into constraints on the neutron-star radius. Based on these constraints, models that predict a stiff symmetry energy, and thus large stellar radii, can be ruled out. Given the sensitivity of the neutron-skin thickness of 208Pb to the symmetry energy, we infer an upper limit on the neutron skin significantly lower than the one reported by the PREX collaboration. However, if the upcoming PREX-II experiment confirms that the neutron-skin thickness of 208Pb is large, this may be evidence in favor of a softening of the symmetry energy at high densities, likely indicative of a phase transition in the interior of neutron stars.