STUDIES OF TMDs AT HERMES

Single-spin asymmetries measured at the HERMES experiment for pions and charged kaons in semi-inclusive deep-inclastic scattering of electrons and positrons off a transversely polarized hydrogen target, are reported. Two of them reveal clear 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.