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A direct measurement of the helicity dependence of the total photoabsorption cross section on the proton was carried out at MAMI (Mainz) in the energy range 200 < Eγ < 800 MeV. The experiment used a 4π 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 γ0 determined from the data are 226±5(stat) ± 12(sys) µb and -187±8(stat) ± 10(sys) 10 −6 fm 4 , respectively, for 200 < Eγ < 800 MeV. PACS number(s): 13.60. Le, 13.60.Hb, 25.20.Lj

The helicity dependence of the single pion photoproduction on the proton has been measured in the energy range from 200 to 450 MeV for the first time. The experiment, performed at the Mainz microtron MAMI, used a 4pi-detector system, a circularly polarized, tagged photon beam, and a frozen-spin target. The data obtained provide new information for multipole analyses of pion photoproduction and determine the main contributions to the Gerasimov-Drell-Hearn sum rule and the forward spin polarizability gamma(0).

To verify the fundamental Gerasimov-Drell-Hearn (GDH) sum rule for the first time experimentally, we measured the helicity dependent total photoabsorption cross section with circularly polarized real photons and longitudinally polarized nucleons in the photon energy range 0.68-1.82 GeV with the tagged photon facility at ELSA. The experiment was carried out with a 4pi detection system, a circularly polarized tagged photon beam, and a frozen spin polarized proton target. The contribution to the GDH sum rule in this photon energy range is [49.9+/-2.4(stat)+/-2.2(syst)] microb.

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