Elastic Proton Form Factor Measurements

College Of William And Mary, Williamsburg VA

Investigators

Abstract

This grant will continue to support the preparation of an important experiment approved at Jefferson Lab (JLab) with highest rating. The experiment will determine the way the charges and currents inside the proton are distributed in space, from the scattering of polarized 6 GeV electrons on a liquid hydrogen target. The equipment is designed to detect the scattered electron in a large lead glass calorimeter, and the polarization of the recoiling proton in a polarimeter. The information is obtained from the degree and spatial orientation of the polarization of the recoiling proton. The novelty of the experiment is that the amount of energy (and momentum) that will be imparted to the proton will be larger than in all previous experiments of this type. Increasing the energy transferred to the target, which here is the nucleon in hydrogen and therefore a proton, gives access to corresponding smaller probing distances inside the proton; at the largest energy transfer of this experiment one is probing the proton on a distance scale equal to 1 twelfth the proton radius. The results of the two previous polarization transfer experiments at JLab are at variance with the results of a long series of measurements of the cross section data for the same process. There is currently much theoretical activity attempting to explain the striking difference between the results from the two techniques. This grant supports my group at the College of William and Mary, which includes one graduate student, one postdoctoral fellow, several occasional undergraduate students and me.