Symmetry aspects of the pion leptoproduction and the upper limit of the   Levelt-Mulders asymmetry
  We examined the symmetry aspect of the semi-inclusive one-pion production in the deep inelastic scattering of a lepton beam off an unpolarized nucleon target, with an emphasis on the positivity restrictions on the corresponding structure functions. In combination with the Callan-Gross-type relation between two twist-two structure functions $W_1$ and $W_2$, we derived an upper bound on the Levelt-Mulders asymmetry, which occurs when the lepton beam is longitudinally polarized. 
Wei Lu, Jian-Jun Yang