SCATTERING OF REAL PHOTONS AND THE STRUCTURE OF BARYONS.
Abstract The leading corrections to point scattering from the proton are characterized by two charge and four spin-polarizability parameters that are sensitive to the proton's internal structure. We review the status of these, focusing on a recent extraction of the backward spin-polarizability, γ π = [−27.1 ± 2.2( stat + sys ) −2.4 +2.8 ( model )] × 10 −4 fm 4 , which differs significantly from theoretical expectations.