The Compton scattering cross section on the proton has been measured at laboratory angles of 90$^\circ$ and 135$^\circ$ using tagged photons in the energy range 70--100 MeV and simultaneously using untagged photons in the range 100--148~MeV. With the aid of dispersion relations, these cross sections were used to extract the electric and magnetic polarizabilities, $\bar{ \alpha}$ and $\bar{ \beta}$ respectively, of the proton. We find $$\bar{ \alpha}+\bar{ \beta} = ( 15.0 \pm 2.9 \pm 1.1 \pm 0.4 ) \times 10^{ -4} \: { \rm fm}^3,$$ in agreement with a model-independent dispersion sum rule, and $$\bar{ \alpha}-\bar{ \beta} = ( 10.8 \pm 1.1 \pm 1.4 \pm 1.0 ) \times 10^{ -4} \: { \rm fm}^3,$$ where the errors shown are statistical, systematic, and model-dependent, respectively. A comparison with previous experiments is given and global values for the polarizabilities are extracted.
Categories: nucl-ex hep-ex