Measurement of beam-spin asymmetries for π+ electroproduction above the baryon resonance region

We report the first evidence for a nonzero beam-spin azimuthal asymmetry in the electroproduction of positive pions in the deep-inelastic kinematic region. Data for the reaction $e\overrightarrow{p}{e}^{\prime }{\pi }^{+}X$ have been obtained using a polarized electron beam of 4.3 GeV with the CEBAF Large Acceptance Spectrometer at the Thomas Jefferson National Accelerator Facility. The amplitude of the $\sin \phi$ modulation increases with the momentum of the pion relative to the virtual photon, z. In the range $z=0.5\mathrm{}–0.8$ the average amplitude is $0.038\pm 0.005\pm 0.003$ for a missing mass $M_X>1.1\mathrm{}\mathrm{GeV}$ and $0.037\pm 0.007\pm 0.004$ for $M_X>1.4\mathrm{}\mathrm{GeV}.$