Azimuthal asymmetry in electro-production of neutral pions in semi-inclusive DIS

Recently HERMES has observed an azimuthal asymmetry AUL in electro-production of neutral pions in semi-inclusive deep-inelastic scattering of unpolarized positrons off longitudinally polarized protons. This asymmetry (like those observed in the production of charged pions) is well reproduced theoretically by using the non-perturbative calculation of the proton transversity distribution h1a in the effective chiral quark–soliton model combined with experimental DELPHI-data on the new T-odd Collins fragmentation function H1⊥. There are no free, adjustable parameters in the analysis. Using the zh-dependence of the HERMES azimuthal asymmetry and the calculated transversity distributions the zh-dependence of the Collins fragmentation function is obtained. The value obtained from HERMES data is consistent with the DELPHI result, even though these results refer to different scales.