Nucleon Polarizabilities from Low-Energy Compton Scattering

An effective field theory is used to give a model-independent description of Compton scattering at energies comparable to the pion mass. The amplitudes for scattering on the proton and the deuteron, calculated to fourth order in small momenta in chiral perturbation theory, contain four undetermined parameters that are in one-to-one correspondence with the nucleon polarizabilities. These polarizabilities are extracted from fits to data on elastic photon scattering on hydrogen and deuterium. For the proton we find: alpha_p = 12.1 +/- 1.1 +/- 0.5, beta_p = 3.4 +/- 1.1 +/- 0.1, both in units of 10^{-4} fm^3. For the isoscalar polarizabilities we obtain: alpha_N = 13.0 +/- 1.9 +3.9/-1.5 (in the same units) while beta_N is consistent with zero, within sizeable error bars.