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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.

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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 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.

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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.

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Animated gif made out of all figures.

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Tables and captions

Summary of relative systematic uncertainties on the total cross-section.

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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, $\epsilon_{\rm single}=0.3329 \pm 0.0003$.

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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, $\epsilon_{\rm single}=0.3329 \pm 0.0003$.

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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 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 $\sigma_{\gamma p\rightarrow J/\psi p(W_-)}$.

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