  We use the Color Glass Condensate (CGC) framework to study the production of
forward heavy quark-antiquark pairs in unpolarized proton-nucleus or
proton-proton collisions in the small-x regime. In the limit of nearly
back-to-back jets, the CGC result simplifies into the transverse-momentum
dependent (TMD) factorization approach. For massless quarks, the TMD
factorization formula involves three unpolarized gluon TMDs: the
Weizs\"{a}cker-Williams gluon distribution, the adjoint-dipole gluon
distribution, and an additional one. When quark masses are kept non-zero, three
new gluon TMDs appear -- each partnered to one of the aforementioned
distributions -- which describe the distribution of linearly-polarized gluons
in the unpolarized small-x target. We show how these six gluon TMDs emerge from
the CGC formulation and we determine their expressions in terms of Wilson line
correlators. We calculate them analytically in the McLerran-Venugopalan model,
and further evolve them towards smaller values of x using a numerical
implementation of JIMWLK evolution.
