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

Abstract

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 $e\stackrel{\ensuremath{\rightarrow}}{p}{e}^{\ensuremath{'}}{\ensuremath{\pi}}^{+}X$ 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 $\mathrm{sin}\ensuremath{\varphi}$ modulation increases with the momentum of the pion relative to the virtual photon, z. In the range $z=0.5--0.8$ the average amplitude is $0.038\ifmmode\pm\else\textpm\fi{}0.005\ifmmode\pm\else\textpm\fi{}0.003$ for a missing mass ${M}_{X}>1.1\mathrm{GeV}$ and $0.037\ifmmode\pm\else\textpm\fi{}0.007\ifmmode\pm\else\textpm\fi{}0.004$ for ${M}_{X}>1.4\mathrm{GeV}.$