Measurement of exclusive $J/\psi$ and $\psi(2S)$ production at $\sqrt{s}=13$ TeV

Measurements are presented of the cross-section for the central exclusive production of J/ψ → µ +µ − and ψ(2S) → µ +µ − processes in proton-proton collisions at p s = 13 TeV with 2016–2018 data. They are performed by requiring both muons to be in the LHCb acceptance (with pseudorapidity 2 < ηµ± < 4.5) and mesons in the rapidity range 2.0 < y < 4.5. The integrated cross-section results are σJ/ψ→µ+µ− (2.0 < yJ/ψ < 4.5, 2.0 < ηµ± < 4.5) = 400 ± 2 ± 5 ± 12 pb , σψ(2S)→µ+µ− (2.0 < yψ(2S) < 4.5, 2.0 < ηµ± < 4.5) = 9.40 ± 0.15 ± 0.13 ± 0.27 pb , where the uncertainties are statistical, systematic and due to the luminosity determination. In addition, a measurement of the ratio of ψ(2S) and J/ψ cross-sections, at an average photon-proton centre-of-mass energy of 1 TeV, is performed, giving σψ(2S) σJ/ψ = 0.1763 ± 0.0029 ± 0.0008 ± 0.0039 , where the first uncertainty is statistical, the second systematic and the third due to the knowledge of the involved branching fractions. For the first time, the dependence of the J/ψ and ψ(2S) cross-sections on the total transverse momentum transfer is determined in p p collisions and is found consistent with the behaviour observed in electron-proton collisions.