Measurement of the Beam-Recoil Polarization in Low-Energy Virtual Compton Scattering from the Proton

Abstract

Double-polarization observables in the reaction $\vec{e}p\to e^{\prime }\overrightarrow{p^{\prime }}{\hat{I}}^3$ 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 $P_{LT}^{\perp }$ is extracted for the first time, with the value $(-15.4\pm 3.3(stat.{)}_{-2.4}^{+1.5}(syst.))Ge{V}^{-2}$, using the low-energy theorem for Virtual Compton Sattering. This structure function provides a hitherto unmeasured linear combination of the generalized polarizabilities of the proton.