Measurement of the beam-recoil polarization in low-energy virtual Compton scattering from the proton

Abstract

Double-polarization observables in the reaction e - p→e′p - ′γ have been measured at Q2=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 PLT is extracted for the first time, with the value (-15.4±3.3(stat.)-2.4+1.5(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.