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 ep⃗e′π+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 sinφ 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±0.005±0.003 for a missing mass MX>1.1GeV and 0.037±0.007±0.004 for MX>1.4GeV.
Physical Review D
Avakian, H.; Biselli, Angela; and CLAS Collaboration, "Measurement of beam-spin asymmetries for π+ electroproduction above the baryon resonance region" (2004). Physics Faculty Publications. 99.
H. Avakian et al. [CLAS Collaboration], "Measurement of beam-spin asymmetries for π+ electroproduction above the baryon resonance region", Physical Review D 69.11 (2004) DOI: 10.1103/PhysRevD.69.112004