Beam Single Spin Asymmetries in Electron-Proton Scattering
Student: Pratik Sachdeva
School: Washington University in St. Louis
Mentored By: Wally Melnitchouk and Peter Blunden
The Q-weak experiment at Jefferson Lab aims to provide a precision test of the Standard Model through electron-proton scattering. In Q-weak, a polarized electron beam was scattered off an unpolarized proton target. The difference between the cross sections of the spin up and spin down cases results in a beam single spin asymmetry (BSSA), which may be longitudinal, transverse, or normal, depending on the polarization of the electrons. There was concern that unconsidered BSSA effects may have introduced a displacement in the measurements of the BSSA. To determine if there was a displacement, we computed the beam transverse single spin asymmetry (BTSSA) that results from the interference of the one-photon and Z exchange amplitudes and compared this to the beam normal single spin asymmetry (BNSSA), which results from the interference of the one- and two-photon exchange amplitudes. It was found that the combination of the BNSSA and the BTSSA results in a phase shift too small to be detectable, implying that the Q-weak measurements for the BNSSA are accurate. The framework we developed could potentially be used to evaluate BSSA in other experimental settings in the near forward limit.