Measurements are reported of the central exclusive production of \jpsi and \psitwos mesons in $pp$ collisions at a centreofmass 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 products of the crosssections and the branching fractions for the decays to dimuons, where both muons are within the pseudorapidity range $2.0<\eta<4.5$, are measured to be $$ \begin{array}{rcl} \sigma_{J/\psi\rightarrow\mu^+\mu^}&=&435 \pm 18 \pm 17 \pm 16 {\rm \ pb},\\ \sigma_{\psi(2S)\rightarrow\mu^+\mu^}&=&11.1 \pm 1.1 \pm 0.3 \pm 0.4 {\rm \ pb}.\\ \end{array} $$ The first uncertainties are statistical, the second are systematic, and the third are due to the luminosity determination. The crosssections are also measured differentially for meson rapidities between 2.0 and 4.5. Good agreement is observed with theoretical predictions. Photoproduction crosssections are derived and compared to previous experiments, and a deviation from a pure powerlaw extrapolation of lower energy data is observed.
Invariant mass distribution of dimuon candidates. The $ { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu}$ and $\psi {(2S)}$ mass windows of the signal regions are indicated by the vertical lines. 
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Transverse momentum squared for dimuons in the nonresonant region. The upper distributions are without any requirement on HeRSCheL : the lower are with the HeRSCheL veto applied. The total fit includes the electromagnetic CEP signal events as described by the LPAIR generator as well as the inelastic background. 
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Distributions, normalised to unit area, of the logarithm of the discriminating variable $\chi_{\rm HRC}^2$ that is related to activity in HeRSCheL . The response to three classes of events, as described in the text, is shown. The selection requirement for the analysis is indicated by the red vertical line and the arrow. 
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Top: transverse momentum squared distribution of (left) $ { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu}$ and (right) $\psi {(2S)}$ candidates when data is below the HeRSCheL threshold. Bottom: CEP signal for the (left) $ { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu}$ and (right) $\psi {(2S)}$ selections. The single exponential fit of the signal is shown by the curve superimposed on the data points. 
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Differential crosssections compared to LO and NLO theory JMRT predictions [28,29] for the $ { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu}$ meson (top) and the $\psi {(2S)}$ meson (bottom). The inner error bar represents the statistical uncertainty; the outer is the total uncertainty. Since the systematic uncertainty for the $\psi {(2S)}$ meson is negligible with respect to the statistical uncertainty, it is almost not visible in the lower figure. 
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Compilation of photoproduction crosssections for various experiments. The upper (lower) plot uses the $ { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu}$ ( $\psi {(2S)}$ ) data. 
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Animated gif made out of all figures. 
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Summary of relative systematic uncertainties on the total crosssection. 
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Tabulation of numbers entering the crosssection calculation for the $ { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu}$ analysis with statistical and systematic uncertainties for the integrated luminosity of $\mathcal{L}_{\rm tot}=204\pm8$ $ pb^{1}$ and the fraction of singleinteraction beam crossings, $\epsilon_{\rm single}=0.3329 \pm 0.0003$. 
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Tabulation of numbers entering the crosssection calculation for the $\psi {(2S)}$ analysis with statistical and systematic uncertainties for the integrated luminosity of $\mathcal{L}_{\rm tot}=204\pm8$ $ pb^{1}$ and the fraction of singleinteraction beam crossings, $\epsilon_{\rm single}=0.3329 \pm 0.0003$. 
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(Top) Statistical and (bottom) systematic correlation matrices for $ { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu}$ , where each column corresponds to one rapidity bin in increasing order. As the matrix is symmetric, only the top triangle is shown. 
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Values used in evaluating the photoproduction crosssection using Eq. 2 for the $ { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu}$ and $\psi {(2S)}$ analysis with gap survival factors for the production of $ { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu}$ and $\psi {(2S)}$ mesons at $\sqrt{s}=13$ TeV [31]. For the $J/\psi$ analyis, $\sigma_{\gamma p\rightarrow J/\psi p(W_+)}$ is calculated using the powerlaw description of HERA or the JMRT NLO description for $\sigma_{\gamma p\rightarrow J/\psi p(W_)}$. 
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Gap survival factors for the production of $ { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu}$ and $\psi {(2S)}$ mesons at $\sqrt{s}=7$ TeV. 
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Created on 20 February 2021.