Compton scattering by the proton

Compton scattering by the proton has been measured over a wide range covering photon energies 250 MeV less than or similar to E-gamma less than or similar to 800 MeV and photon scattering angles 30 degrees less than or similar to theta (lab)(gamma) less than or similar to 150 degrees, 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%27s backward spin polarizability was determined to be gamma pi = (-37.9 +/- 0.6(stat+syst) +/- 3.5(model)) x 10(-4) fm(4). (C) 2001 Elsevier Science B.V. All rights reserved

Measurement of the G asymmetry for the γp→Nπ channels in the Δ(1232) resonance region

The G asymmetry of the gamma p -> N pi reaction has been measured for the first time for E gamma = 340 +/- 14 MeV. This observable, for which very scarce published data exist, plays an important role to disentangle the contributions of the various nucleon resonances. The experiment, performed at the Mainz microtron MAIM, used a 4 pi-detector system, a linearly polarized, tagged photon beam, and a longitudinally polarized proton target

Helicity dependence of the gamma p → N pi channels and multipole analysis in the Delta region

A high-quality double-polarization data set for the helicity dependence of the total and differential cross-sections for both gammap --> Npi channels in the Delta region has been obtained in the framework of the GDH experiment. The experiment, performed at the Mainz microtron MAMI, used a 4pi detection system, a circularly polarized photon beam, and a longitudinally polarized frozen-spin target. These data are included in the database to perform a multipole analysis to determine the properties of the Delta(1232)-resonance. For the resonant Delta(1232) multipoles we find a very good agreement with previous analyses, while the nonresonant ones show significant deviations

Experimental check of the Gerasimov-Drell-Hearn sum rule for H-1

For the first time we checked the fundamental Gerasimov-Drell-Hearn (GDH) sum rule for the proton experimentally in the photon energy range from 0.2-2.9 GeV with the tagged photon facilities at MAMI (Mainz) and ELSA (Bonn). New data of the doubly polarized total cross section difference are presented in the energy range from 1.6 to 2.9 GeV. The contribution to the GDH integral from 0.2-2.9 GeV yields [254+/-5(stat)+/-12(syst)] mub with negative contributions in the Regge regime at photon energies above 2.1 GeV. This trend supports the validity of the GDH sum rule

First measurement of the Gerasimov-Drell-Hearn integral for H-1 from 200 to 800 MeV

A direct measurement of the helicity dependence of the total photoabsorption cross section an the proton was carried out at MAMI (Mainz) in the energy range 200 detection system, a circularly polarized. tagged photon beam, and a frozen spin target. The contributions to the Gerasimov-Drell-Hearn sum rule and to the forward spin polarizability gamma (0) determined from the data are 226 +/- 5(stat) +/- 12(syst) mub and -187 +/- 8(stat) +/- 10(syst) x 10(-6) fm(4), respectively, for 200 < 800 MeV

Intermediate resonance excitation in the gamma p → p pi(0)pi(0) reaction

The helicity dependence of the total cross section for the (gamma) over right arrow(p) over right arrow -> p pi(0)pi(0) reaction has been measured for the first time at incident photon energies from 400 to 800 MeV. The measurement, performed at the tagged photon beam facility of the MAMI accelerator in Mainz, used the large acceptance detector DAPHNE and a longitudinally polarized frozen-spin target. This channel is found to be excited predominantly when the photon and proton have a parallel spin orientation, most likely due to the intermediate production of the D-13(1520) resonance. However, the contribution of the antiparallel spin configuration, arising from other reaction mechanisms, is also not negligible. This result gives important new information to resolve the existing model discrepancies in the identification of the nucleon resonances contributing to this channel. (c) 2005 Elsevier B.V. All rights reserved

First measurement of the helicity-dependent (gamma)over-right-arrow(p)over-right-arrow → p eta differential cross-section

The helicity dependence of the (gamma) over right arrow(p) over right arrow -+ peta reaction has been measured for the first time at a center-of-mass angle theta(eta)*= 70degrees in the photon energy range from 780 MeV to 790 MeV. The experiment, performed at the Mainz microtron MAMI, used a 4pi-detector system, a circularly polarized, tagged photon beam. and a longitudinally polarized frozen-spin target. The helicity 3/2 cross-section is found to be small and the results for helicity 1/2 agree with predictions from the MAID analysis