Fixed-t subtracted dispersion relations for Compton scattering off the nucleon - art. no. 015204

Fixed-t subtracted dispersion relations iue presented for Compton scatterin g off the nucleon at energies E gamma less than or equal to 500 MeV, as a f ormalism to extract the nucleon polarizabilities with a minimum of model de pendence. The subtracted dispersion integrals are mainly saturated by pi N intermediate states in the s channel, gamma N --> pi N --> gamma N, and pi pi intermediate states in the t channel, gamma gamma --> pi pi --> N (N) ov er bar. For the subprocess gamma gamma --> pi pi, we construct a unitarized amplitude and find a good description of the available data. We show resul ts for Compton scattering using the subtracted dispersion relations and dis play the sensitivity on the scalar polarizability difference alpha-beta and the backward spin polarizability gamma(pi), which enter directly as fit pa rameters in the present formalism. Double polarization observables are show n to have a unique potential for measuring the spin polarizabilities of the nucleon.