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

Measurements are presented of the cross-section for the central exclusive production of $J\mathrm{/}\psi \to {\mu }^{+}{\mu }^{-}$ and $\psi (2S)\to {\mu }^{+}{\mu }^{-}$ processes in proton-proton collisions at $\sqrt{s}=13$ TeV with 2016-2018 data. They are performed by requiring both muons to be in the LHCb acceptance (with pseudorapidity $2<{\eta }_{{\mu }^{\pm }}<4.5$) and mesons in the rapidity range $2.0<y<4.5$. The integrated cross-section results are \begin{equation*} σ_{J/ψ\toμ^+μ^-}(2.0<y_{J/ψ}<4.5,2.0<η_{μ^\pm} < 4.5) = 400 \pm 2 \pm 5 \pm 12 ,{\rm pb},, \end{equation*} \begin{equation*} σ_{ψ(2S)\toμ^+μ^-}(2.0<y_{ψ(2S)}<4.5,2.0<η_{μ^\pm} < 4.5) = 9.40 \pm 0.15 \pm 0.13 \pm 0.27 ,{\rm pb},, \end{equation*} where the uncertainties are statistical, systematic and due to the luminosity determination. In addition, a measurement of the ratio of $\psi (2S)$ and $J\mathrm{/}\psi$ cross-sections, at an average photon-proton centre-of-mass energy of 1 TeV, is performed, giving \begin{equation*} \frac{σ_{ψ(2S)}}{σ_{J/ψ}} = 0.1763 \pm 0.0029 \pm 0.0008 \pm 0.0039 ,, \end{equation*} 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 $J\mathrm{/}\psi$ and $\psi (2S)$ cross-sections on the total transverse momentum transfer is determined in $pp$ collisions and is found consistent with the behaviour observed in electron-proton collisions.