The Super Bigbite Spectrometer physics program

Abstract

The structure of the nucleon is a central problem in strong interaction physics in the non-perturbative regime. Indeed, the vast majority of the known matter in the Universe is made of protons and neutrons which are a remarkable emergent phenomenon of quantum chromodynamics. A critical aspect of investigating nucleon structure experimentally is the measurement of fundamental quantities such the elastic nucleon form factors. Also important is the measurement transverse momentum dependent distribution functions. Accessing such quantities experimentally, however, is challenging because of the small cross sections involved, particularly at high momentum transfer. We present here a physics program that is addressing this challenge based on the Super Bigbite Spectrometer (SBS) that has recently been built at the Thomas Jefferson National Accelerator Facility. SBS provides a relatively large solid angle of 70 msr and can be used at high luminosities and forward-scattering angles. It is based on a single large dipole magnet in an open-geometry in which the detector package has a direct line of sight to the target. This approach is only possible through the use of detector technology that can operate at very high rates while providing excellent spatial resolution. It is the product of solid angle and luminosity that is critical when measuring small cross sections, and in this regard, among spectrometer systems at JLab, SBS is presently unique in its capability. The first set of experiments utilizing SBS has been successfully completed, and more experiments are planned for the future. We also discuss a proposed upgrade that would increase the SBS solid angle to 260 msr, thereby opening perspectives for an even broader physic program.