Very low $x$ gluon density determined by LHCb exclusive $J\mathrm{/}\psi$ data

The low $x$ behaviour of the gluon density $xg(x,{\mu }^2)$ at scale ${\mu }^2=2.4$ GeV${}^2$ is determined using exclusive $J\mathrm{/}\psi$ production data from HERA and LHCb within the framework of collinear factorisation at next-to-leading order (NLO). It is shown that in the interval $3\times {10}^{-6}<x<{10}^{-3}$ the gluon distribution function grows as $xg(x,{\mu }^2)\propto x^{-\lambda }$ with $\lambda =0.135\pm 0.006$. The impact this experimental data will have for the global parton distribution function (PDF) analyses in this low $x$ domain is quantified. No indication in favour of parton density saturation is observed.