  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.
