NNLL fit of the transverse-momentum-dependent distribution of unpolarised gluons to LHCb data on $J/ψ$-pair production

Abstract: We perform the first fit of the unpolarised gluon transverse-momentum-dependent distribution $f_1<sup>g$ up to next-to-next-to-leading-logarithmic accuracy to the double-differential distribution in the invariant mass and transverse momentum of the latest $J/ψ$-pair production LHCb data. We review previous attempts to describe the older $J/ψ$-pair LHCb data using $f_1<sup>g$, and discuss two novel methodologies which allow us to account in the fit for both the perturbative and PDF uncertainties to construct $f_1<sup>g$ in the perturbative region. It turns out that the consideration of these uncertainties is crucial for a meaningful fit to such low-scale gluon-sensitive data.