The Pb Radius Experiment (PREX)

TL;DR

The PREX experiment measured the parity-violating asymmetry in elastic electron scattering from lead-208 to determine the neutron distribution radius, providing the first electroweak observation of the neutron skin in a heavy nucleus.

Contribution

This work presents the first measurement of parity-violating asymmetry in electron-lead scattering, offering a model-independent probe of neutron distribution in nuclei.

Findings

Measured APV = 0.656 +/- 0.060(stat) +/- 0.014(syst) ppm

Determined neutron skin thickness Rn - Rp = 0.33 +16 -18 fm

First electroweak observation of neutron skin in a heavy nucleus

Abstract

We report the first measurement of the parity-violating asymmetry APV in the elastic scattering of polarized electrons from 208Pb from the Lead Radius Experiment PREX which ran in Hall A at the Thomas Jefferson National Accelerator Facility (JLab). APV is sensitive to the radius of the neutron distribution Rn. The Z boson that mediates the weak neutral interaction couples mainly to neutrons and provides a clean, model-independent measurement of the RMS radius Rn of the neutron distribution in the nucleus and is a fundamental test of nuclear structure theory. The result, APV = 0.656 +/- 0.060(stat) +/- 0.014(syst) ppm, corresponds to a difference between the radii of the neutron and proton distributions Rn - Rp = 0.33 +16 -18 fm and provides the first electroweak observation of the neutron skin which is expected in a heavy, neutron-rich nucleus.