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.

Compton scattering by the proton has been measured over a wide range covering photon energies 250 MeV < ∼ E γ < ∼ 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 data are in good agreement with the dispersion theory based on the SAID-SM99K parameterization of photo-meson amplitudes. From the subset of data between 280 and 360 MeV the resonance pion-photoproduction amplitudes were evaluated leading to the multipole E2/M1 ratio EMR(340 MeV) = (−1.6 ± 0.4 stat+syst ± 0.2 model )%. From all data below 455 MeV the proton's backward spin polarizability was determined to be γ π = (−37.9 ± 0.6 stat+syst ± 3.5 model ) × 10 −4 fm 4 .

New precision measurements of cross sections and polarization asymmetries in the p͑ g, gp͒ reaction at 90 ± c.m. are presented for incident energies between 213 and 333 MeV. A long-standing problem with earlier experiments that appeared to violate unitarity at the peak of the D is resolved. Data are compared to theories based on baryon resonance structure and to dispersion relations. Recent calculations using the proton polarizabilities are closest to the data, although inconsistencies are observed near the D resonance. PACS numbers: 13.60.Fz, 13.88.+e, 14.20.Gk, 25.20.Dc Elastic (Compton) scattering of intermediate energy photons from the proton is a potentially rich source of structure information. It is sensitive to the proton's electric (a) and magnetic (b) polarizabilities [1], to the deformation of the nucleon through the electric gND coupling [2], and even to the sign of the p 0 decay constant, F p [3,4]. There are many published calculations for Compton scattering. These can be grouped into two general categories, those based on the baryon resonance spectrum or its underlying quark structure [5][6][7], and those relying upon unitarity and dispersion relations to phenomenologically describe elastic scattering in terms of photopion production [3,4,8-10]. These calculations give significantly different predictions, particularly in the region of the D resonance.A number of measurements of proton Compton scattering have been reported [1,[11][12][13][14][15][16], and several authors of dispersion calculations have pointed out a significant inconsistency between many of these experiments and p photoproduction data near the peak of the D [3,8-10]. Compton scattering can be described by six independent amplitudes. Their imaginary parts can be calculated from (g, p) multipoles using s-and u-channel unitarity, and dispersion integrals can be written for their real parts. Four of these integrals converge rapidly with energy. However, the remaining two, those involving a photon helicity flip, do not converge rapidly, making subtractions essential. One of these is dominated by t-channel p 0 exchange, the Low amplitude [17], and can be readily evaluated in terms of the p 0 lifetime. However, the other contains contributions from multiple meson exchange in the t channel that are quite poorly determined. In principle, the beam-polarization asymmetry constrains this other amplitude [4]. But prior to our new measurements, only a single datum with large errors had been published for this observable [18]. Alternatively, sum rules can be used to write the subtraction function for this spin-flip amplitude in terms of the difference of the proton polarizabilities, a 2 b, which can then be fixed by fitting a perturbative expansion of the cross section to data below the p threshold [3,4]. Although this provides a good description of scattering below the D [1,19], the peak cross sections appear to be overestimated [3,4].A lower unitarity bound on the Compton cross sections, which avoids the uncertainties of the dispersion calcul...

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