Study of Exclusive Pion Photoproduction from a Nucleon in the Wide Angle Regime: The A$_{LL}$ and K$_{LL}$ experiments

The measured differential cross sections for the single neutral/charge pion photoproduction from a nucleon in the wide angle regime (s,-t,-u much larger hadronic scale) disagree with theoretical calculations by {f almost two orders of magnitude} in spite of many years of investigation.

A solution was proposed by P.~Kroll and K. Passek-Kumericki, whose GPD-based theory includes both twist-2 and twist-3 amplitudes. The signatures of the twist-3 amplitude (compared to twist-2) is a increase in the cross section and has opposite signs of the double polarization observables K$_{LL}$ and A$_{LL}$ at backward production angles. An experimental check of the prediction would provide valuable information on the validity of the handbag mechanism in the GPD framework in the accessible energy range.\\ An approved experiment E12-21-005 measures the double spin asymmetry A$_{LL}$~for charged pion photoproduction in the wide angle regime using the reaction ($vec gamma vec n ightarrow pi^- p$). This measurement will provide essential complimentary information to the K$_{LL}$~measurement in experiment E12-20-008 approved by PAC48 in 2020. These experiments will be performed in Hall A at Jefferson Lab as a part of the Super Bigbite Spectrometer (SBS) series of experiments. For measuring K$_{LL}$, we will use the 6.6 GeV CEBAF electron beam to impinge photons in the energy range E$_gamma ge 4.0$ GeV on a deuterium target where as for measuring A$_{LL}$, a polarized electron and a polarized $^{3}$He target will be utilized. Physics overview and projected results will be presented in this talk.