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

TL;DR

This paper analyzes recent COMPASS and HERMES measurements of the $ ext{cos} 2 ext{phi}$ asymmetry in unpolarized SIDIS, providing evidence for the Boer-Mulders effect through a phenomenological fit.

Contribution

It offers the first experimental evidence of the Boer-Mulders effect in SIDIS by fitting the asymmetry data with a theoretically consistent Boer-Mulders function.

Findings

Good fit of the data with a Boer-Mulders function aligned with theoretical expectations.

First experimental evidence of the Boer-Mulders effect in SIDIS.

Analysis distinguishes between leading-twist and twist-4 contributions.

Abstract

We present a phenomenological analysis of the $\cos 2 \phi$ 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 $h_1^{\perp}(x, k_T^2)$, and the twist-4 Cahn contribution, involving unpolarized transverse-momentum distribution functions. We show that a reasonably good fit of the data 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.