A new result from the PREX-2 experiment, recently published in Physical Review Letters, suggests that an unexpectedly thick “skin” of neutrons surrounds the lead-208 nucleus.  With 82 protons and 126 neutrons, it has long been thought that the neutron distribution in this nucleus must be slightly larger than the proton distribution. But while the proton distribution had been well-mapped using electron scattering, measurements of the electrically-neutral neutrons required required a new experimental approach

The problem of mapping out the distribution of neutrons in a nucleus was solved by using the weak interaction between the electron and nucleus. A coincidence of nature leaves the weak charge of the proton to be small, less than 1/12^th that of the neutron. And while the weak interaction is about a million times weaker than the electromagnetic interaction, it can be clearly identified as the only interaction which will violate mirror symmetry. Making a precise measurement of this broken mirror symmetry is experimentally difficult, but provides a measure of the neutron distribution which is free from theoretical uncertainties in interpretation. 

The experiment, which ran at Jefferson Lab in the summer of 2019, measured an asymmetry in the rate of elastic scattering that correlated to the helicity of electrons incident on the lead nucleus.  This small asymmetry, about one part in two million, was measured to a precision of about 3% of itself, and it implied that the neutrons were distributed in a sphere that was 5% +/- 1.3% larger than that of the proton distribution. This suggests a larger than expected penalty for increasing the density of neutron rich matter.  A companion paper, published simultaneously, explores the implication of this result for the density and nature of neutron stars.  The PREX-2 result, extrapolated to the few-times higher density of a neutron star, is seen to be in good agreement with the 2018 measurement of the density of a neutron star by the NICER collaboration, but in mild tension with the tidal deformability measured in the 2017 observation of a neutron star inspiral by LIGO. 

The paper was selected for a Viewpoint in Physics and as an Physical Review Letters Editors’ Suggestion, and has been written up in a handful of science and technology magazines such as Science, Science News, and Gizmodo.  The PREX collaboration is led by UVa professor Kent Paschke and includes UVa professors Nilanga Liyange, Xiaochao Zheng, and Gordon Cates. The collaboration also includes six UVa graduate students, three of whom are writing PhD dissertations on the experiment. 

Prof. Paschke was interviewed by Salon about the results and how it relates to neutron stars.