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We study the $\cos 2 \phi$ azimuthal asymmetry in unpolarized semi-inclusiveDIS, taking into account both the perturbative contribution (gluon emission andsplitting) and the non perturbative effects arising from intrinsic transversemotion and transverse spin of quarks. In particular we explore the possibilityto extract from $<\cos 2 \phi>$ some information about the Boer--Muldersfunction $h_1^{\perp}$, which represents a transverse--polarization asymmetryof quarks inside an unpolarized hadron. Predictions are presented for theHERMES, COMPASS and JLab kinematics, where $<\cos 2 \phi>$ is dominated by thekinematical higher--twist contribution, and turns to be of order of fewpercent. We show that a larger asymmetry in $\pi^-$ production, compared to$\pi^+$ production, would represent a signature of the Boer--Mulders effect.

A systematic phenomenological study of the $\cos 2 \phi$ asymmetry in unpolarized semi--inclusive DIS