Request for Beam Time to Complete PREX

This document addresses the beam time required to achieve the design sensitivity of the PREX experiment. As we elaborate below, a 1% measurement of the neutron radius using an electroweak probe will remain compelling for years to come. We estimate that the recently completed physics run of the PREX experiment will yield a ∼2.5% measurement of the neutron radius. While this is a major accomplishment as a first measurement of its kind, it will have a relatively minor impact in the broader context of nuclear structure physics and nuclear astrophysics, unless future running with 208Pb can provide a determination approaching 1%. We summarize the accomplishments of the run and the lessons learned. We address the issue of a potential followup measurement, and describe the ancitipated improvements that should allow us to take data at a rate equivalent to that in the original proposal estimate, and thus achieve a 1% measurement of the neutron radius in about 25 PAC days. 1 Theoretical Context 1.1 Parity-violation in Elastic Electron Scattering off 208Pb The parity-violating asymmetry ALR in elastic scattering from 208Pb is sensitive to the neutron radius Rn and is a clean method for measuring this parameter. This idea was suggested some years ago [1] for plane waves. The basic concept is that the neutral weak boson, the Z0, couples mainly to neutrons in the same way that the photon couples to the protons. Then the electromagnetic(weak) amplitude is proportional to the proton(neutron) form factor Fp(n)(Q) = 1 4π ∫ drj0(Qr)ρp(n)(r) and the asymmetry is given in the Born approximation by ALR = GFQ 4πα √ 2 [ 4 sin2 θW − 1 + Fn(Q) Fp(Q2) ]