Azimuthal distributions of charged hadrons, pions, and kaons produced in   deep-inelastic scattering off unpolarized protons and deuterons
  The azimuthal cos{\phi} and cos2{\phi} modulations of the distribution of hadrons produced in unpolarized semi-inclusive deep-inelastic scattering of electrons and positrons off hydrogen and deuterium targets have been measured in the HERMES experiment. For the first time these modulations were determined in a four-dimensional kinematic space for positively and negatively charged pions and kaons separately, as well as for unidentified hadrons. These azimuthal dependences are sensitive to the transverse motion and polarization of the quarks within the nucleon via, e.g., the Cahn, Boer-Mulders and Collins effects. 
HERMES Collaboration: A. Airapetian, N. Akopov, Z. Akopov, E. C.   Aschenauer, W. Augustyniak, R. Avakian, A. Avetissian, E....
hep-ex nucl-ex