Neutron skins and neutron stars

Abstract

Abstract. The neutron-skin thickness of heavy nuclei provides a fundamental link to the equation of state of neutron-rich matter, and hence to the properties of neutron stars. The Lead Radius Experiment ("PREX") at Jefferson Laboratory has recently provided the first model-independence evidence on the existence of a neutron-rich skin in 208 Pb. In this contribution we examine how the increased accuracy in the determination of neutron skins expected from the commissioning of intense polarized electron beams may impact the physics of neutron stars.Keywords: Neutron distributions, neutron stars PACS: 21.10. Gv, 21.65.Cd, 25.30.Bf, 26.60.Kp NEUTRON SKINSThe Lead Radius EXperiment ("PREX") at the Jefferson Laboratory has used parity-violating electron scattering to probe the weak-charge density of 208 Pb [1,2]. Given that the weak charge of the neutron is much larger than that of the proton, parity-violating electron scattering provides a clean probe of neutron densities that is free from strong-interaction uncertainties [3]. By invoking some mild assumptions, PREX provided the first largely modelindependent determination of the neutron radius (R n ) of 208 Pb. Since charge radii of stable nuclei are known with exquisite accuracy [4], PREX effectively provided the first clean evidence in favor of a neutron-rich skin in 208 Pb [1, 2]:(1)Although PREX demonstrated excellent control of systematic errors, unforeseen technical problems resulted in time losses that significantly compromised the statistical accuracy of the measurement. Fortunately, the PREX collaboration has made a successful proposal for additional beam time, so that the original 1% goal (±0.06 fm) may be attained [5]. Given that PREX demonstrated that model-independent measurements of weak-charge densities are now feasible, it is pertinent to ask whether a measurement in a different neutron-rich nucleus could prove advantageous. Indeed, the case of 48 Ca seems particularly attractive for several reasons. First, 48 Ca is a doubly-magic nucleus that is already within the reach of ab-initio calculations [6]. Thus, the Calcium Radius EXperiment ("CREX") could provide a critical bridge between ab-initio approaches and density-functional theory, that remains as the sole theoretical alternative for the calculation of the properties of medium to heavy nuclei. Second, by providing this kind of bridge, CREX will help elucidate the character of the three-nucleon force, which plays a critical role in determining the limits of nuclear existence. Finally, CREX together with PREX-II will provide calibrated anchors for hadronic measurement of neutron skins at radioactive beam facilities. In essence, PREX-II in combination with CREX will place much-needed constrains on the poorly-known isovector sector of the nuclear energy density functional, namely, that sector in which protons and neutrons contribute with opposite sign. Note that a typical example of such a quantity is the isovector density ρ 1 = ρ p −ρ n which-unlike the isoscalar density ρ 0 = ρ p +ρ n -is fair...

Impact of form factor uncertainties on interpretations of coherent elastic neutrino-nucleus scattering data

“…The rate for core-collapse supernova in units of Mpc −3 year −1 is determined by the star formation ratė ρ * (z) and the initial mass function Ψ(M) as The parameters in Eq. (29) are closely related to those in Ref. [10], but involve a q 2 dependence related to the proton and neutron form factors.…”

“…Pairing correlations and deformation become relevant for nuclei that are not doubly-magic. The situation is worse for nuclei with unpaired nucleons like 133 Cs, 127 I and 129 Xe, in which case calculations assume that the nuclei are even-even nuclei (although they are not), and rescale the occupancy of the valence orbital by a suitable factor with the hope that bulk properties like the weak radius are not sensitive to this "spherical approximation" [29].…”

Neutron skins of atomic nuclei: per aspera ad astra

“…To improve the nuclear description beyond the simplistic Fermi distributions used in earlier studies, ground state proton and neutron densities have been furnished for most tin isotopes using density functional theory (DFT). Whereas all functionals reproduce the binding energy and charge radii of magic and semi-magic nuclei, they nevertheless generate a wide range of values for both the symmetry energy and its slope at saturation density [104]. The theoretical predictions displayed in Fig.…”

“…Figure 17. Neutron-skin thickness for some tin isotopes, extracted from experiments using hadronic probes [53,88,56,68,105], are compared against theoretical calculations with density functionals that predict a different behavior for the value (J) and slope (L) of the symmetry energy at saturation density (full lines) [104].…”

Pygmy resonances and symmetry energy

“…To cite a few of these works, we list Refs. [78,79,80,81,82,83,84,85,86,87,88,89,90,91,92]. It is therefore clear that pygmy resonances constitute a new phenomenon in nuclear physics which appeared in the context of neutron-rich nuclei and the effect is also enhanced in nuclei close to the drip-line with small binding energies.…”