Very low $x$ gluon density determined by LHCb exclusive $J/ψ$ data

The low $x$ behaviour of the gluon density $xg(x,μ^2)$ at scale $μ^2=2.4$ GeV$^2$ is determined using exclusive $J/ψ$ 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 $xg(x,μ^2)\propto x^{-λ}$ with $λ=0.135\pm 0.006$. 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.

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