Single-spin asymmetries for semi-inclusive electroproduction of charged pions in deep-inelastic scattering of positrons are measured for the first time with transverse target polarization. The asymmetry depends on the azimuthal angles of both the pion (phi) and the target spin axis (phi(S)) about the virtual-photon direction and relative to the lepton scattering plane. The extracted Fourier component sin((phi+phi(S))(pi)(UT) is a signal of the previously unmeasured quark transversity distribution, in conjunction with the Collins fragmentation function, also unknown. The component sin((phi-phi(S)(pi)(UT) arises from a correlation between the transverse polarization of the target nucleon and the intrinsic transverse momentum of quarks, as represented by the previously unmeasured Sivers distribution function. Evidence for both signals is observed, but the Sivers asymmetry may be affected by exclusive vector meson production.

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We report on precision measurements of the elastic cross section for electron-proton scattering performed in Hall C at Jefferson Lab. The measurements were made at 28 distinct kinematic settings covering a range in momentum transfer of 0.4 < Q 2 < 5.5 (GeV/c) 2 . These measurements represent a significant contribution to the world's cross section data set in the Q 2 range where a large discrepancy currently exists between the ratio of electric to magnetic proton form factors extracted from previous cross section measurements and that recently measured via polarization transfer in Hall A at Jefferson Lab. This data set shows good agreement with previous cross section measurements, indicating that if a here-to-fore unknown systematic error does exist in the cross section measurements then it is intrinsic to all such measurements.

Spin transfer in deep-inelastic ⌳ electroproduction has been studied with the HERMES detector using the 27.6 GeV polarized positron beam in the DESY HERA storage ring. For an average fractional energy transfer ͗z͘ϭ0.45, the longitudinal spin transfer from the virtual photon to the ⌳ has been extracted. The spin transfer along the ⌳ momentum direction is found to be 0.11Ϯ0.17(stat)Ϯ0.03(syst); similar values are found for other possible choices for the longitudinal spin direction of the ⌳. This result is the most precise value obtained to date from deep-inelastic scattering with charged lepton beams, and is sensitive to polarized up quark fragmentation to hyperon states. The experimental result is found to be in general agreement with various models of the ⌳ spin content, and is consistent with the assumption of helicity conservation in the fragmentation process.

Single-spin asymmetries have been measured for semi-inclusive electroproduction of pi(+), pi(-), pi(0) and K+ mesons in deep-inelastic scattering off a longitudinally polarised deuterium target. The asymmetries appear in the distribution of the hadrons in the azimuthal angle phi around the virtual photon direction, relative to the lepton scattering plane. The corresponding analysing powers in the sinphi moment of the cross section are 0.012 +/- 0.002(stat.) +/- 0.002(syst.) for pi(+), 0.006 +/- 0.003(stat.) +/- 0.002(syst.) for pi(-), 0.021 +/- 0.005(stat.) +/- 0.003(syst.) for pi(0) and 0.013 +/- 0.006(stat.) +/- 0.003(syst.) for K The sin 20 moments are compatible with zero for all particles. (C) 2003 Elsevier Science B.V All rights reserved