Transverse momentum dependence of the quark helicity distributions and the Cahn effect in double-spin asymmetry A{sub LL} in semiinclusive DIS

Within the LO QCD parton model of Semi-Inclusive Deep Inelastic Scattering, lN{yields}lhX, with unintegrated quark distribution and fragmentation functions, we study the P{sub hT} dependence of the double longitudinal-spin asymmetry A{sub LL}. We include 1/Q kinematic corrections, which induce an azimuthal modulation of the asymmetry, analogous to the Cahn effect in unpolarized SIDIS. We show that a study of A{sub LL} and of the weighted DSA A{sub LL}{sup cos{phi}{sub h}} allows to extract the transverse momentum dependence of the unintegrated helicity distribution function g{sub 1L}{sup q}(x,k{sub perpendicular}) [or {delta}q(x,k{sub perpendicular})]. Predictions, based on some models for the unknown functions, are given for ongoing COMPASS, HERMES and JLab experiments.