Single Spin Azimuthal Asymmetries and Transversity

Single Spin azimuthal Asymmetries (SSA) in semi-inclusive deep-inelastic lepton-nucleon scattering can be generally produced by time reversal odd (T-odd) distribution or fragmentation functions. In the case of an unpolarized positron beam and a longitudinally or transversely polarized target, SSAs can have two origins: the chiral odd transversity distribution h1 in conjunction with the chiral odd Collins fragmentation function H1perpendicular and the T-odd Sivers distribution function f1Tperpendicular in conjunction with a spin-independent fragmentation function. In the case of a longitudinally polarized target both mechanisms contribute to the same asymmetry, whereas in case of transverse target polarisation they give rise to different angular dependences and can therefore be measured separately. The present status of HERMES data taking with a transversely polarized proton target will be presented