Studies of gluon TMDs and their evolution using quarkonium-pair production at the LHC

$$J/\psi $$ J / ψ - or $$\Upsilon $$ Υ -pair production at the LHC are promising processes to study the gluon transverse momentum distributions (TMDs) which remain very poorly known. In this article, we improve on previous results by including the TMD evolution in the computation of the observables such as the pair-transverse-momentum spectrum and asymmetries arising from the linear polarization of gluons inside unpolarized protons. We show that the azimuthal asymmetries generated by the gluon polarization are reduced compared to the tree level case but are still of measurable size (in the 5–10% range). Such asymmetries should be measurable in the available data sets of $$J/\psi $$ J / ψ pairs and in the future data sets of the high-luminosity LHC for $$\Upsilon $$ Υ pairs.