Implications of PREX-2 on the Equation of State of Neutron-Rich Matter.

Laboratory experiments sensitive to the equation of state of neutron rich matter in the vicinity of nuclear saturation density provide the first rung in a "density ladder" that connects terrestrial experiments to astronomical observations. In this context, the neutron skin thickness of ^{208}Pb (R_{skin}^{208}) provides a stringent laboratory constraint on the density dependence of the symmetry energy. In turn, an improved value of R_{skin}^{208} has been reported recently by the PREX collaboration. Exploiting the strong correlation between R_{skin}^{208} and the slope of the symmetry energy L within a specific class of relativistic energy density functionals, we report a value of L=(106±37) MeV-which systematically overestimates current limits based on both theoretical approaches and experimental measurements. The impact of such a stiff symmetry energy on some critical neutron-star observables is also examined.

Neutron Skins and Neutron Stars in the Multimessenger Era.

The historical first detection of a binary neutron star merger by the LIGO-Virgo Collaboration [B. P. Abbott et al., Phys. Rev. Lett. 119, 161101 (2017)PRLTAO0031-900710.1103/PhysRevLett.119.161101] is providing fundamental new insights into the astrophysical site for the r process and on the nature of dense matter. A set of realistic models of the equation of state (EOS) that yield an accurate description of the properties of finite nuclei, support neutron stars of two solar masses, and provide a Lorentz covariant extrapolation to dense matter are used to confront its predictions against tidal polarizabilities extracted from the gravitational-wave data. Given the sensitivity of the gravitational-wave signal to the underlying EOS, limits on the tidal polarizability inferred from the observation 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. Indeed, we deduce an upper limit on the radius of a 1.4M_{⊙} neutron star of R_{⋆}^{1.4}<13.76 km. Given the sensitivity of the neutron-skin thickness of ^{208}Pb to the symmetry energy, albeit at a lower density, we infer a corresponding upper limit of about R_{skin}^{208}≲0.25 fm. However, if the upcoming PREX-II experiment measures a significantly thicker skin, this may be evidence of a softening of the symmetry energy at high densities-likely indicative of a phase transition in the interior of neutron stars.

Compactness of Neutron Stars.

Recent progress in the determination of both masses and radii of neutron stars is starting to place stringent constraints on the dense matter equation of state. In particular, new theoretical developments together with improved statistical tools seem to favor stellar radii that are significantly smaller than those predicted by models using purely nucleonic equations of state. Given that the underlying equation of state must also account for the observation of 2M⊙ neutron stars, theoretical approaches to the study of the dense matter equation of state are facing serious challenges. In response to this challenge, we compute the underlying equation of state associated with an assumed mass-radius template similar to the "common radius" assumption used in recent studies. Once such a mass-radius template is adopted, the equation of state follows directly from the implementation of Lindblom's algorithm; assumptions on the nature or composition of the dense stellar core are not required. By analyzing mass-radius profiles with a maximum mass consistent with observation and common radii in the 8-11 km range, a lower limit on the stellar radius of a 1.4M⊙ neutron star of RNS≳10.7 km is required to prevent the equation of state from violating causality.

Has a thick neutron skin in 208Pb been ruled out?

The Lead Radius Experiment has provided the first model-independent evidence in favor of a neutron-rich skin in 208Pb. Although the error bars are large, the reported large central value of 0.33 fm is particularly intriguing. To test whether such a thick neutron skin in 208Pb is already incompatible with laboratory experiments or astrophysical observations, we employ relativistic models with neutron-skin thickness in 208Pb ranging from 0.16 to 0.33 fm to compute ground-state properties of finite nuclei, their collective monopole and dipole response, and mass-versus-radius relations for neutron stars. No compelling reason was found to rule out models with large neutron skins in 208Pb from the set of observables considered in this Letter.

Reduction of the spin-orbit splittings at the n = 28 shell closure.

The N = 28 shell closure has been investigated via the 46Ar(d,p)47Ar transfer reaction in inverse kinematics. Energies and spectroscopic factors of the neutron p(3/2), p(1/2), and f(5/2) states in 47Ar were determined and compared to those of the 49Ca isotone. We deduced a reduction of the N = 28 gap by 330(90) keV and spin-orbit weakenings of approximately 10(2) and 45(10)% for the f and p states, respectively. Such large variations for the f and p spin-orbit splittings could be accounted for by the proton-neutron tensor force and by the density dependence of the spin-orbit interaction, respectively. This contrasts with the picture of the spin-orbit interaction as a surface term only.

Neutron-rich nuclei and neutron stars: a new accurately calibrated interaction for the study of neutron-rich matter.

An accurately calibrated relativistic parametrization is introduced to compute the ground state properties of finite nuclei, their linear response, and the structure of neutron stars. While similar in spirit to the successful NL3 parameter set, it produces an equation of state that is considerably softer--both for symmetric nuclear matter and for the symmetry energy. This softening appears to be required for an accurate description of several collective modes having different neutron-to-proton ratios. Among the predictions of this model are a symmetric nuclear-matter incompressibility of K=230 MeV and a neutron skin thickness in 208 Pb of Rn-Rp=0.21 fm. The impact of such a softening on various neutron-star properties is also examined.

Neutron star structure and the neutron radius of 208Pb.

We study relationships between the neutron-rich skin of a heavy nucleus and the properties of neutron-star crusts. Relativistic effective field theories with a thicker neutron skin in 208Pb have a larger electron fraction and a lower liquid-to-solid transition density for neutron-rich matter. These properties are determined by the density dependence of the symmetry energy which we vary by adding nonlinear couplings between isoscalar and isovector mesons. An accurate measurement of the neutron radius in 208Pb-via parity violating electron scattering-may have important implications for the structure of the crust of neutron stars.