Transverse spin phenomena in semi-inclusive deep-inelastic scattering of electrons and positrons off polarized hydrogen

Six azimuthal asymmetry amplitudes, measured for pions and charged kaons in semi-inclusive deep-inelastic scattering of electrons and positrons off a transversely polarized hydrogen target, are reported. Two of them reveal evidences of non-zero leading-twist parton distribution functions, such as the poorly known transversity and Sivers functions, describing crucial aspects of the dynamics of quarks in a transversely polarized nucleon. Evidences of a non-zero favored and disfavored Collins fragmentation function with opposite sign are also observed. These measurements constitute either preliminary or final results from the HERMES Collaboration. A first extraction of acceptance-free Ph⊥-weighed Collins and Sivers amplitudes, based on a private analysis of the HERMES data, is also reported. The Sivers polarization, also based on a private analysis, was extracted from the Ph⊥-weighted Sivers amplitudes. It is compatible with orbital angular momenta of opposite signs for u and d quarks in a transversely polarized proton.