H. W. Griesshammer (Technical University Munich)

Model-Independent Extraction of Nucleon Polarisabilities from Compton Scattering off Nucleons and Deuterons

I show that one can accurately determine the nucleon's polarisabilities directly from Compton scattering off the proton and deuteron below the Delta-resonance. A multipole analysis of the amplitudes reveals the dispersive effects from the internal degrees of freedom of the nucleon: Whereas the coupling to the pion cloud suffices to describe data up to 70 MeV, the energy-dependence of the polarisabilities induced by the Delta(1232)-resonance is strong around the one-pion production threshold and proves indispensible to understand in particular Deuteron Compton scattering at 95 MeV as measured at SAL. Predicting this energy-dependence by Chiral Perturbation Theory with the Delta as explicit degree of freedom, one finds the proton electric polarizability = 11.0(1.4) x 10^-4 fm^3 and the magnetic susceptibility = 2.8(1.4) x 10^-4 fm^3, which compares favourably both in magnitude and error with alternative extractions. For the static isoscalar electric dipole polarisability one finds 12.4(2.0) x 10^-4 fm^3, and for the corresponding magnetic susceptibility 2.1(2.0) x 10^-4 fm^3. Therefore, proton and neutron polarisabilities are identical within the accuracy of available data. We propose also to extract the thus far ill-determined spin-dependent polarisabilities from experiments in a model-independent way, and present predictions.