QCD predictions for the azimuthal asymmetry in charm leptoproduction for the COMPASS kinematics

Abstract We present the QCD predictions for the azimuthal cos ⁡ 2 φ asymmetry in charm leptoproduction for the kinematics of the COMPASS experiment at CERN. The asymmetry is predicted to be large, about 15%. The radiative corrections to the QCD predictions for the cos ⁡ 2 φ distribution are estimated to be small, less than 10%. Our calculations show that the azimuthal asymmetry in charm production is well defined in pQCD: it is stable both perturbatively and parametrically, and practically insensitive to theoretical uncertainties in the input parameters. We analyze the nonperturbative contributions to the cos ⁡ 2 φ distribution due to the gluon transverse motion in the target and the c -quark fragmentation. Because of the c -quark low mass, the nonperturbative contributions are expected to be sizable, about (30–40)%. We conclude that extraction of the azimuthal asymmetries from available COMPASS data will provide valuable information about the transverse momentum dependent distribution of the gluon in the proton and the c -quark hadronization mechanism. Finally, we discuss the cos ⁡ 2 φ asymmetry as a probe of the gluonic analogue of the Boer–Mulders function, h 1 ⊥ g , describing the linear polarization of gluons inside unpolarized proton.