E04-108: Measurement of GEp/GMp to Q2=9 GeV2 via recoil polarization
E04-019: Measurement of the Two-Photon Exchange Contribution in ep Elastic Scattering Using Recoil Polarization
There are two ways to measure form factors. In both, high-energy electrons are �scattered� from targets like the proton; this scattering is equivalent to viewing an object invisible to the naked eye with a powerful microscope. The first of these methods, the polarization observables technique, will be used in the forthcoming experiment. The polarization technique measures the amount of spin polarization that the incoming electron transfers to the proton as a result of the collision between the electron and the proton.
Form factor measurements using the polarization technique are thought to be more accurate compared to a second method, cross section measurements. Cross section measurements determine the probability that the incoming electrons will be deflected by a given angle after scattering from the proton.
Cross section measurements may not be as accurate, because they are affected by secondary processes; the polarization technique is only weakly affected by these secondary processes. That may explain a discrepancy between results from cross section vs. polarization measurements. A second experiment, E04-019, will run interlaced with E04-108 and is designed to help identify the source of the discrepancy. It is a polarization measurement of the form factors over a range of electron-scattering angles and energy at constant proton momentum.