Transverse momentum dependence of the quark helicity distributions and the Cahn effect in double-spin asymmetry $A_{LL}$ in Semi Inclusive DIS

Abstract

Within the LO QCD parton model of Semi Inclusive Deep Inelastic Scattering, $\ell N\to \ell hX$, with unintegrated quark distribution and fragmentation functions, we study the ${\text{bfP}}_{hT}$ dependence of the double longitudinal-spin asymmetry $A_{LL}$. 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 $A_{LL}$ and of the weighted DSA $A_{LL}^{\cos {\varphi }_h}$ allows to extract the transverse momentum dependence of the unintegrated helicity distribution function $g_{1L}^q(x,k_{\perp })$ [or $\Delta q(x,k_{\perp })]$. Predictions, based on some models for the unknown functions, are given for ongoing COMPASS, HERMES and JLab experiments.

Comments: Replaced with published in PHYSICAL REVIEW D 74, 074015 (2006) version