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.

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)] microb with negative contributions in the Regge regime at photon energies above 2.1 GeV. This trend supports the validity of the GDH sum rule.

No abstract

No abstract

The asymmetric emission of R~Sntgenbremsstrahlung produced in the scattering of transversely polarized electrons by Au-atoms was measured at an electron energy of E =mc2/4 for photon emission angles 0=60 ~ 100 ~ 145 ~ and compared to calculations of this process based on a partial waves method.The influence of electron spin on the production of bremsstrahlung has been a major point of interest in numerous investigations about this process. As a consequence of spin orbit interaction in analogy to Mott-scattering one expects an azimuthal asymmetry in the spatial distribution of the bremsstrahlung around the direction of the incident electrons if these are transversely polarized with their polarization vector perpendicular to the production plane. Observing only the produced photons and disregarding the scattered electrons this asymmetry effect can be described following a formalism introduced by Tseng and Pratt [1] who contributed detailed calculations to atomic field bremsstrahlung based on a partial waves method. The asymmetric emission of the bremsstrahlung produced by transversely polarized electrons is expressed by the cross section for the angular distribution of the emitted photons which is given by:where: do-0=differential cross section averaged over the initial electron spins and summed over the final photon and electron polarization Z = atomic number of the target E =initial kinetic energy of the electrons * Present address: Physics Department, Yale University, New Haven CT 06520, USA k = energy of the produced photons 0=photon emission angle ~=P.s-% with P --polarization degree of incident electrons s = direction of initial electron spin %=unit vector perpendicular to the production plane defined by the momenta of the emitted photon and incident electron.The correlation coefficient C20 between initial electron and final photon polarization vanishes in the low Z and high E limit. Experimental results for C2o published to date [2][3][4] are not a stringent test of the validity of theoretical predictions. In part they disagree with theory or show only moderate statistical significance. So it seemed worthwhile to perform supplementary measurements with comparable parameters to clarify the picture. As can be seen from the above formula C2o can be deduced from: a) measurements of the cross section for emission angles 0 and -0 symmetric to the direction of the incident electrons in a plane perpendicular to the electron spin, b) measurements of the cross section for a fixed emission angle but with opposite orientation of the electron spin. Both experimental procedures change the sign of ~ while keeping all other parameters unaltered 9 In the present experiment method (b) was followed using a beam with the polarization of the electrons alternating.