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

TL;DR

This paper demonstrates that quarkonium-pair production in unpolarised proton collisions is the optimal process to measure linearly-polarised gluon distributions via azimuthal asymmetries, with existing data showing significant asymmetries.

Contribution

It identifies di-J/psi production as the best process for measuring linearly-polarised gluons and performs the first fit of the unpolarised TMD gluon distribution using recent LHCb data.

Findings

Di-J/psi production shows azimuthal asymmetries up to 50%.

Existing data can be used to extract gluon polarization information.

First fit of unpolarised TMD gluon distribution from LHCb data.

Abstract

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.