Pinning down the linearly-polarised gluons inside unpolarised protons using quarkonium-pair production at the LHC.

We show that the production of J/psi or Upsilon pairs in unpolarised pp collisions is currently the best process to measure the momentum-distribution of linearly-polarised gluons inside unpolarised protons through the study of azimuthal asymmetries. Not only the short-distance coefficients for such reactions induce the largest possible cos 4 phi modulations, but analysed data are already available. Among the various final states previously studied in unpolarised pp collisions within the TMD factorisation approach, di-J/psi production exhibits by far the largest asymmetries, up to 50% in the region studied by the ATLAS and CMS experiments. In addition, we use the very recent LHCb data at 13 TeV to perform the first fit of the unpolarised transverse-momentum-dependent gluon distribution.