A continuation of the measurement of the proton electric and magnetic form factor ratio, GEp/GMp, to at least 12 GeV squared four-momentum transfer squared, will become possible after the JLab upgrade to twice the present electron beam energy. Likewise, measurements of the two form factors of the neutron, GEn and GMn to much larger four momentum squared will become possible. Three approved nucleon form factor experiments are the prime justification for the construction of a new setup in Hall A at JLab, the Super Bigbite Spectrometer (SBS), to be funded by DOE. The SBS will be a large acceptance device, which will be used in different configurations; for the GEp/GMp experiment it will consist of a large dipole, followed by a set of gas electron multiplier chambers (GEM) to define the track of the recoil proton. A new double polarimeter will be built, consisting of two polyethylene analyzer blocks, each followed by a set of GEMs. The new GEp/GMp experiment will also require the use of a reconfigured BigCal lead glass calorimeter (as was used recently in the GEp(III) experiment), to detect the electron; a double plane of GEM detectors, to be built and assembled in part at William and Mary, will be added to improve the BigCal position resolution.
Investigating the detailed structure of proton and neutron is one of several major goals towards unraveling the structure of matter; both particles are the most numerous in the visible universe. The proton has a size and a structure. It is not just a dimensionless point in space, but also it has evanescent components called sea-quarks, which are always created in pairs (quark-antiquark). Quarks interact with each other via the exchange of gluons, which are the carriers of the strong force. At JLab protons and neutrons are probed by the scattering of electrons. For a modest shock the electron is only aware of the overall size and shape of the proton. As the strength of the shock increases, the region of internal space probed becomes smaller. Ultimately, if one keeps increasing the strength of the shock, the electron starts to see the individual quarks, and that is the interest of this kind of investigation. With a new postdoctoral fellow, we will be able to contribute to the many tasks required to build a new experimental setup which will play an important role in future investigations of the structure of the proton. Such a position offers a recent physics PhD holder the occasion to get in-depth, hands-on experience in all phases of research activity, and take responsibility: a prerequisite for entering a teaching and/or research position at a University, a National research Lab or industry.
