Single-spin azimuthal asymmetry in exclusive electroproduction of π+ mesons

A single-spin asymmetry in the distribution of exclusively produced π+ mesons azimuthally around the virtual photon direction relative to the lepton scattering plane has been measured for the first time in deep-inelastic scattering of positrons off longitudinally polarized protons. Integrated over the experimental acceptance, the sinφ moment of the polarization asymmetry of the cross section is measured to be −0.18±0.05(stat.)±0.02(syst.). The asymmetry is also studied as a function of the relevant kinematic variables, and its magnitude is found to grow with decreasing x and increasing −t and vanish at t→tmin (where x is the Bjorken scaling variable and t is the squared four-momentum transferred to the nucleon).