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

Double-polarization observables in the reaction of photon electroproduction with beam and recoil target polarization 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 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.