Q2 Measurement and Challenges in PREX

Abstract

The four-momentum transfer squared (Q2) is an important kinematic quantity for nuclear experiments. The importance of the Q2 measurement was emphasized in the case of PREX, conducted at Hall A, Jefferson Lab, which measured the parity-violating electroweak asymmetry in the elastic scattering of polarized electrons off a 208Pb target against an average Q2 in order to find the neutron radius. With the requirement of 1% uncertainty in Q2, the measurement became extremely challenging with the need to find new solutions and techniques to overcome several constraints including the use of a low electron beam current, the limited capabilities of the existing equipment and the design of the experiment (a 5^o central scattering angle). With a new technique using the idea of differential nuclear recoil for elastic electron scattering off target nuclei of different masses to measure the spectrometer central angle (θ0) and scattering angle (θ) for Q2 calculation, the average Q2 was measured to be 0.00906±0.00009 GeV2, which was within the 1% uncertainty requirement of the experiment. This article summarizes all procedures and challenges of Q2 measurement in PREX and possible improvement for future PREX-like experiments.