Elastic Proton Form Factor Measurements

College Of William And Mary, Williamsburg VA

Investigators

Abstract

The first experiment to measure the ratio of the elastic and magnetic form factors of the proton at large four-momentum squared, Q^2 with good accuracy produced unexpected results. This experiment realized in 1998 at Jefferson Lab used the recoil polarization technique, which takes advantage of the fact that the transverse component of the recoil proton polarization is proportional to the product of the 2 form factors, thus allowing accurate determination of the small electric form factor. In a second experiment at JLab in 2000, the measurement was extended to 5.6 GeV^2 , the results confirmed the intrinsically different Q^2-dependence of the electric and magnetic form factors of the proton beyond any doubt. The data from these 2 JLab experiments have been a challenge for theoretical models which attempt to describe the proton structure in the non-perturbative domain of Quantum Chromo Dynamics (QCD). As the simplicity of perturbative QCD may not be encountered at accessible laboratory energies, an understanding of the structure of the proton (and likewise of the other nucleon, the neutron) requires the use of phenomenological models which depend in parts on data. Nucleons are the building blocks of the matter we are made of and in which we live. A full understanding of the structure of the nucleon in terms of its quark and gluon constituents will be achieved only when we accumulate a complete data base of its response to the electromagnetic probe, electrons and photons, over a large enough Q^2-range; such experiment represent the main part of the research program at JLab. We now have a third experiment, recently approved with highest priority; it will extend the range of Q^2 probed to 9 GeV^2.This grant will make it possible for me to continue leading this project to and through the next phase, which require new detectors (a new polarimeter to measure the recoil proton polarization, and a new calorimeter to measure the associate scattered electron) and will take 3 years to prepare.