World’s First Calcium Transverse Beam Asymmetry Measurement

Dr. Dustin McNulty
Department of Physics
Idaho State University

The PREX-II and CREX collaborations at Jefferson Lab, Newport News Virginia, have recently measured the parity-conserving beam-normal single-spin asymmetry (or transverse beam asymmetry: A_n) for several nuclear targets, including first ever measurements on ⁴⁰Ca and ⁴⁸Ca.  These measurements were auxiliary, meaning they were not the main motivating measurement for the experiments; they were performed as a means to bound any potential contamination from the process to the main physics measurement (the parity-violating asymmetry: A_PV).  To first order, or in the Born approximation, A_n vanishes due to time-reversal invariance, however in reality A_n is tiny, but non zero, and can be relatively easily measured by parity violating electron scattering experiments such as PREX-II and CREX.   There is often much theoretical interest in A_n measurements since they give direct access to higher order scattering effects such as from two-photon exchange amplitudes—which are largely unmeasured and unknown.  This talk will introduce the motivations for the measurements along with the apparatus and techniques.  Preliminary results will be given for new An measurements on ¹²C, ²⁰⁸Pb, ⁴⁰Ca, and ⁴⁸Ca; they will be presented with previously published measurements from PREX-I as well as with recent theoretical predictions.