Central exclusive production of $J/\psi$ and $\psi(2S)$ mesons in pp collisions at $\sqrt{s}=13$ TeV

A measurement is made of the central exclusive production of $J/\psi$ and $\psi(2S)$ mesons in $pp$ collisions at a centre-of-mass energy of 13 TeV. Backgrounds are significantly reduced compared to previous measurements made at lower energies through the use of new forward shower counters. The cross-section times branching fractions for the decays to dimuons, where both muons are within the pseudorapidity region $2<\eta<4.5$, are measured to be $$ \begin{array}{rcl} \sigma_{J/\psi\rightarrow\mu^+\mu^-}(2.0<\eta_{\mu^+},\eta{\mu^-}<4.5)&=&407\pm 8\pm 24 \pm 16 {\rm \ pb} \\ \sigma_{\psi(2S)\rightarrow\mu^+\mu^-}(2.0<\eta_{\mu^+},\eta{\mu^-}<4.5)&=&9.4\pm 0.9\pm 0.6 \pm 0.4 {\rm \ pb.}\\ \end{array} $$ where the first uncertainty is statistical, the second is systematic, and the third is due to the luminosity determination. The cross-sections are also measured differentially in ten bins of pseudorapidity for the $J/\psi$ meson and in three bins for the $\psi(2S)$. Good agreement is observed with theoretical predictions. Photoproduction cross-sections are derived and compared to previous experiments; a deviation from a pure power-law extrapolation of lower energy data is observed.