Contribution of exclusive diffractive processes to the measured azimuthal asymmetries in SIDIS

TL;DR

This paper investigates how exclusive diffractive processes influence azimuthal asymmetries in SIDIS measurements, developing a subtraction method to clarify intrinsic quark transverse momentum effects and Boer-Mulders function contributions.

Contribution

A new method to evaluate and subtract the impact of exclusive diffractive processes from SIDIS azimuthal asymmetry measurements is introduced.

Findings

Exclusive diffractive processes significantly distort measured azimuthal asymmetries.

Subtracting these contributions clarifies the kinematic dependence of asymmetries.

Remaining non-zero asymmetries suggest a non-zero Boer-Mulders function.

Abstract

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.