The Boer-Mulders effect in unpolarized SIDIS: an analysis of the COMPASS and HERMES data on the $\cos 2 \phi$ asymmetry

摘要

We present a phenomenological analysis of the $\cos 2 \phi$ asymmetryrecently measured by the COMPASS and HERMES collaborations in unpolarizedsemi-inclusive deep inelastic scattering. In the kinematical regimes exploredby these experiments the asymmetry arises from transverse-spin and intrinsictransverse-momentum effects. We consider the leading-twist contribution,related to the so-called Boer-Mulders transverse-polarization distribution$h_1^{\perp}(x, k_T^2)$, and the twist-4 Cahn contribution, involvingunpolarized transverse-momentum distribution functions. We show that areasonably good fit of the data is achieved with a Boer-Mulders functionconsistent with the main theoretical expectations. Our conclusion is that theCOMPASS and HERMES measurements represent the first experimental evidence ofthe Boer-Mulders effect in SIDIS.