The transversity function and double spin asymmetry in semi-inclusive DIS

We show that the spin asymmetry in semi-inclusive deep inelastic scattering between a longitudinally polarized lepton beam and a transversely polarized proton target is sensitive to the proton transversity function. The asymmetry, which is twist 3, is estimated to be more than 10% under the most favourable conditions. The experiment we suggest is feasible at facilities like DESY, CERN and JLAB.