Transverse momentum dependent gluon distributions at the LHC

Linearly polarized gluons inside an unpolarized proton contribute to the transverse momentum distributions of (pseudo)scalar particles produced in hadronic collisions, such as Higgs bosons and quarkonia with even charge conjugation ($\eta_c$, $\eta_b$, $\chi_{c0}$, $\chi_{b0}$). Moreover, they can produce azimuthal asymmetries in the associated production of a photon and a $J/\psi$ or a $\Upsilon$ particle, in a kinematic configuration in which they are almost back to back. These observables, which can be measured in the running experiments at the LHC, could lead to a first extraction of both the polarized and the unpolarized gluon distributions and allow for a study of their process and energy scale dependences.