Neutron Skins and Halo Orbits in the $sd$ and $pf$ shells

TL;DR

This paper investigates the mechanisms influencing neutron skin thickness and nuclear radii in $sd$ and $pf$ shell nuclei, highlighting the roles of isovector monopole polarizability and halo orbits, with implications for nuclear structure understanding.

Contribution

It introduces the first detection of halo orbit influence on neutron skins and nuclear radii, expanding understanding of nuclear size variations beyond traditional models.

Findings

Neutron skin estimates align with previous models and recent measurements.

Halo orbits significantly impact isotope shifts and radius constancy in certain nuclei.

A new mechanism involving halo orbits is proposed, possibly extending beyond Efimov physics.

Abstract

The strong dependence of Coulomb energies on nuclear radii makes it possible to extract the latter from calculations of the former. The resulting estimates of neutron skins indicate that two mechanisms are involved. The first one---isovector monopole polarizability---amounts to noting that when a particle is added to a system it drives the radii of neutrons and protons in different directions, tending to equalize the radii of both fluids independently of the neutron excess. This mechanism is well understood and the Duflo-Zuker (small) neutron skin values derived 14 years ago are consistent with recent measures and estimates. The alternative mechanism involves halo orbits whose huge sizes tend to make the neutron skins larger and have a subtle influence on the radial behavior of $sd$ and $pf$ shell nuclei. In particular, they account for the sudden rise in the isotope shifts of nuclei beyond $N=28$ and the near constancy of radii in the $A=40-56$ region. This mechanism, detected here for the first time, is not well understood and may well go beyond Efimov physics usually associated to halo orbits.