Compton scattering from the free and bound proton at backward angles above π-threshold Original Research Article

Differential cross sections for Compton scattering from the free proton at Θγ′lab=130.7° in the energy region from 200 MeV to 410 MeV and for quasi-free Compton scattering from the proton bound in the deuteron at Θγ′lab=148.8° in the energy region from 200 MeV to 290 MeV have been measured. The free proton data are in agreement with dispersion-theory predictions based on standard parameters. The difference of the proton polarizabilities has been extracted from the quasi-free data. Our result, View the MathML source, is in reasonable agreement with the world average of the free proton data if the backward spin polarizability γπ is taken to be View the MathML source as predicted by dispersion theory in agreement with many theoretical calculations. This implies that quasi-free Compton scattering may also be used to determine the electromagnetic polarizabilities of the neutron. No indication has been found of a recently suggested new contribution to γπ.