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

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## Abstract

Measurements are reported of the central exclusive production of \jpsi and \psitwos 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 products of the cross-sections 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 cross-sections are also measured differentially for meson rapidities between 2.0 and 4.5. Good agreement is observed with theoretical predictions. Photoproduction cross-sections are derived and compared to previous experiments, and a deviation from a pure power-law extrapolation of lower energy data is observed.

## Figures and captions

 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. Fig1.pdf [35 KiB] HiDef png [237 KiB] Thumbnail [78 KiB] *.C file 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. Fig2.pdf [26 KiB] HiDef png [253 KiB] Thumbnail [79 KiB] *.C file 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. Fig3.pdf [16 KiB] HiDef png [221 KiB] Thumbnail [75 KiB] *.C file 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. Fig4a.pdf [17 KiB] HiDef png [163 KiB] Thumbnail [65 KiB] *.C file Fig4b.pdf [14 KiB] HiDef png [132 KiB] Thumbnail [54 KiB] *.C file Fig4c.pdf [17 KiB] HiDef png [146 KiB] Thumbnail [54 KiB] *.C file Fig4d.pdf [15 KiB] HiDef png [132 KiB] Thumbnail [50 KiB] *.C file Differential cross-sections 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. Fig5a.pdf [18 KiB] HiDef png [166 KiB] Thumbnail [58 KiB] *.C file Fig5b.pdf [17 KiB] HiDef png [165 KiB] Thumbnail [60 KiB] *.C file Compilation of photoproduction cross-sections for various experiments. The upper (lower) plot uses the ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ ( $\psi {(2S)}$ ) data. Fig6a.pdf [32 KiB] HiDef png [202 KiB] Thumbnail [62 KiB] *.C file Fig6b.pdf [21 KiB] HiDef png [177 KiB] Thumbnail [56 KiB] *.C file Animated gif made out of all figures. PAPER-2018-011.gif Thumbnail

## Tables and captions

 Summary of relative systematic uncertainties on the total cross-section. Table_1.pdf [44 KiB] HiDef png [79 KiB] Thumbnail [11 KiB] tex code Tabulation of numbers entering the cross-section 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 single-interaction beam crossings, $\epsilon_{\rm single}=0.3329 \pm 0.0003$. Table_2.pdf [39 KiB] HiDef png [214 KiB] Thumbnail [32 KiB] tex code Tabulation of numbers entering the cross-section 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 single-interaction beam crossings, $\epsilon_{\rm single}=0.3329 \pm 0.0003$. Table_3.pdf [80 KiB] HiDef png [50 KiB] Thumbnail [7 KiB] tex code (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. Table_5.pdf [11 KiB] HiDef png [36 KiB] Thumbnail [5 KiB] tex code Values used in evaluating the photo-production cross-section 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 power-law description of HERA or the JMRT NLO description for $\sigma_{\gamma p\rightarrow J/\psi p(W_-)}$. Table_7.pdf [72 KiB] HiDef png [122 KiB] Thumbnail [17 KiB] tex code Gap survival factors for the production of ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ and $\psi {(2S)}$ mesons at $\sqrt{s}=7$ TeV. Table_8.pdf [42 KiB] HiDef png [116 KiB] Thumbnail [17 KiB] tex code

Created on 09 December 2018.Citation count from INSPIRE on 09 December 2018.