  Hadron leptoproduction in Semi-Inclusive measurements of Deep-Inelastic
Scattering (SIDIS) on unpolarised nucleons allows one to get information on the
intrinsic transverse momentum of quarks in a nucleon and on the Boer-Mulders
function through the measurement of azimuthal modulations in the cross section.
These modulations were recently measured by the HERMES experiment at DESY on
proton and deuteron targets, and by the COMPASS experiment using the CERN SPS
muon beam and a $^6$LiD target. In both cases, the amplitudes of the
$\cos\phi_h$ and $\cos 2\phi_h$ modulations show strong kinematic dependences
for both positive and negative hadrons. It has been known since some time that
the measured final-state hadrons in those SIDIS experiments receive a
contribution from exclusive diffractive production of vector mesons,
particularly important at large values of $z$, the fraction of the virtual
photon energy carried by the hadron. In previous measurements of azimuthal
asymmetries this contribution was not taken into account, because it was not
known that it could distort the azimuthal modulations. Presently, a method to
evaluate the contribution of the exclusive reactions to the azimuthal
asymmetries measured by COMPASS has been developed. The subtraction of this
contribution results in a better understanding of the kinematic effects, and
the remaining non-zero $\cos 2\phi_h$ modulation gives indication for a
non-zero Boer-Mulders effect.
