Compton scattering by the proton using a large-acceptance arrangement

Abstract:

Compton scattering by the proton has been measured over a wide range covering photon energies 250MeV E _γ 800MeV and photon scattering angles 30^° 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 m _σ≈ 600 MeV. The asymptotic part of the spin-dependent amplitude is found to be well represented by π⁰ exchange in the t-channel. No indications of additional effects were found. Using the mass parameter m _σ 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±0.7model ×10^-4fm³, as obtained from a recent experiment on proton Compton scattering below pion photoproduction threshold, a backward spin polarizability of γ_π = -37.1±0.6stat+syst±3.0model ×10^-4fm⁴ 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↦Δ radiative transition of EMR(340 MeV) = (- 1.7±0.4_stat+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±0.4_stat+syst±0.2_model)% is obtained.