No abstract

The beam-helicity asymmetry has been measured simultaneously for the reactions → e p → e p γ and → e p → e p π 0 in the ∆(1232) resonance region at Q 2 = 0.35 (GeV/c) 2 . The experiment was performed at MAMI with a longitudinally polarized beam and an out-of-plane detection of the proton. The results are compared with calculations based on Dispersion Relations for virtual Compton scattering and with the MAID model for pion electroproduction. There is an overall good agreement between experiment and theoretical calculations. The remaining discrepancies may be ascribed to an imperfect parametrization of some γ ( * ) N → πN multipoles, mainly contributing to the non-resonant background. The beam-helicity asymmetry in both channels (γ and π 0 ) shows a good sensitivity to these multipoles and should allow future improvement in their parametrization. PACS. 13.40.-f Electromagnetic processes and properties -13.60.Fz Elastic and Compton scattering -13.60.Le Meson production -14.20.Gk Baryon resonances with S=0

Double-polarization observables in the reaction ep → e ′ p ′ γ have been measured at Q 2 = 0.33 (GeV/c) 2 . The experiment was performed at the spectrometer setup of the A1 Collaboration using the 855 MeV polarized electron beam provided by the Mainz Microtron (MAMI) and a recoil proton polarimeter. From the double-polarization observables the structure function P ⊥ LT is extracted for the first time, with the value (−15.4 ±3.3 (stat.) +1.5 −2.4 (syst.) ) GeV −2 , using the low-energy theorem for Virtual Compton Scattering. This structure function provides a hitherto unmeasured linear combination of the generalized polarizabilities of the proton.

This paper describes the Monte Carlo simulation developed specifically for the VCS experiments below pion threshold that have been performed at MAMI and JLab. This simulation generates events according to the (Bethe-Heitler + Born) crosssection behaviour and takes into account all relevant resolution-deteriorating effects. It determines the "effective" solid angle for the various experimental settings which are used for the precise determination of the photon electroproduction absolute cross section.