We present our recent calculations on the production of pseudoscalar charmonia eta_c(1S) and eta_c(2S) in the k_T factorization approach. We discuss the regions of longitudinal and transverse momenta of gluons probed in the kinematics of the LHCb or ATLAS experiments.
The crucial ingredient is the off-shell matrix element, which in this case is
related to a g^g^eta_c form factor. Its calculation proceeds analogously to the gamma^ gamma^ eta_c form factor obtained in terms of the light front wave function of the charmonium in .
We compare our results to recent LHCb data and discuss the sensitivity
to the choice of the unintegrated gluon distribution.
 I.~Babiarz, V.~P.~Goncalves, R.~Pasechnik, W.~Schäfer and A.~Szczurek,
Phys. Rev. D 100 (2019) no.5, 054018