We discuss how quarkonium production, in particular $\eta_c$ production in proton-proton collisions, can be used to access gluon transverse-momentum-dependent parton distribution functions (TMDPDFs).
To do so, we apply the effective field theory machinery to factorize the process in terms of gluon TMDPDFs at low transverse momentum, and match this result with the collinear framework to obtain the full transverse-momentum spectrum.
This matching is performed by applying the newly devised inverse-error weighting method, based on an estimation of the uncertainties coming from power corrections to construct a weighted average of both factorization theorems.