Compton scattering by the proton has been measured over a wide range covering photon energies 250 MeVEγ 800 MeV and photon scattering angles 30 • θ lab γ 150 • , using the tagged-photon facility at MAMI (Mainz) and the large-acceptance arrangement LARA. The previously existing data base on proton Compton scattering is greatly enlarged by more than 700 new data points. The new data are interpreted in terms of dispersion theory based on the SAID-SM99K parametrization of photo-meson amplitudes. It is found that two-pion exchange in the t-channel is needed for a description of the data in the second resonance region. The data are well represented if this channel is modeled by a single pole with the mass parameter mσ ≈ 600 MeV. The asymptotic part of the spin dependent amplitude is found to be well represented by π 0 -exchange in the t-channel. No indications of additional effects were found. Using the mass parameter mσ of the two-pion exchange determined from the second resonance region and using the new global average for the difference of the electric and magnetic polarizabilities of the proton, α − β = (10.5 ± 0.9stat+syst ± 0.7 model ) × 10 −4 fm 3 , as obtained from a recent experiment on proton Compton scattering below pion photoproduction threshold, a backward spin-polarizability of γπ = (−37.1 ± 0.6stat+syst ± 3.0 model ) × 10 −4 fm 4 has been determined from data of the first resonance region below 455 MeV. This value is in a good agreement with predictions of dispersion relations and chiral perturbation theory. From a subset of data between 280 and 360 MeV the resonance pion-photoproduction amplitudes were evaluated leading to a E2/M1 multipole ratio of the p → ∆ radiative transition of EMR(340 MeV)= (−1.7±0.4stat+syst ±0.2 model )%. It was found that this number is dependent on the parameterization of photo-meson amplitudes. With the MAID2K parameterization an E2/M1 multipole ratio of EMR(340 MeV)= (−2.0 ± 0.4stat+syst ± 0.2 model )% is obtained.

Two aspects of the nucleon electromagnetic polarizabilities, αN and βN, are discussed. The first one is the interrelation between static and Compton polarizabilities. Relativistically consistent treatment of the polarization effects based on effective Lagrangians results in the conclusion that the Compton polarizabilities, rather than the static ones, drive the energy shift of the nucleon in external fields. The cluster substructure of the static polarizabilities is shown to be crucial for consistent calculations of the polarizabilities. The second aspect concerns the dominant role of pion degrees of freedom in αN as seen through dispersion relations, chiral perturbation theory, and constituent models. The reported calculations of αN within soliton models are shown to be inconsistent with gauge invariance. The difficulties in explaining the observed large diamagnetic component of βN are emphasized.

We present a calculation of the virtual Compton scattering amplitude for γ * + π → γ + π in the framework of chiral perturbation theory at O(p 4 ). We explicitly derive expressions for generalized electromagnetic polarizabilities and discuss alternative definitions of these quantities.

Measuring Compton scattered photons and recoil neutrons in coincidence, quasifree Compton scattering by the neutron has been investigated at MAMI (Mainz) at theta(lab)(gamma) = 136 degrees in an energy range from 200 to 400 MeV. From the data a polarizability difference of alpha(n)-beta(n) = 9.8+/-3.6(stat)+2.1-1.1(syst)+/-2.2(model) in units of 10(-4) fm(3) has been determined. In combination with the polarizability sum alpha(n)+beta(n) = 15.2+/-0.5 deduced from photoabsorption data, the first precise results for the neutron electric and magnetic polarizabilities, alpha(n) = 12.5+/-1.8(stat)+1.1-0.6(syst)+/-1.1(model) and beta(n) = 2.7-/+1.8(stat)+0.6-1.1(syst)-/+1.1(model), are obtained.

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