Boer-Mulders effect in unpolarized SIDIS: An analysis of the COMPASS and HERMES data on the cos2φ asymmetry

We present a phenomenological analysis of the cos2 phi asymmetry recently measured by the COMPASS and HERMES collaborations in unpolarized semi-inclusive deep inelastic scattering. In the kinematical regimes explored by these experiments the asymmetry arises from transverse-spin and intrinsic transverse-momentum effects. We consider the leading-twist contribution, related to the so-called Boer-Mulders transverse-polarization distribution h(1)(perpendicular to)(x, k(2)(T)), and the twist-4 Cahn contribution, involving unpolarized transverse-momentum distribution functions. We show that a reasonably good fit of the preliminary data sets from COMPASS and HERMES is achieved with a Boer-Mulders function consistent with the main theoretical expectations. Our conclusion is that the COMPASS and HERMES measurements represent the first experimental evidence of the Boer-Mulders effect in SIDIS.