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Exclusive J/$\psi$ and $\psi$(2S) production in $pp$ collisions at $\sqrt{s} = 7$ TeV

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Abstract

Exclusive $J/\psi$ and $\psi(2S)$ vector meson production has been observed in the dimuon channel using the LHCb detector. The cross-section times branching fractions to two muons with pseudorapidities between 2.0 and 4.5 are measured to be $ \sigma_{pp\rightarrow J/\psi (\rightarrow \mu^{+} \mu^{-})} (2.0 <\eta_{\mu^{\pm}}< 4.5) = 307 \pm 21 \pm 36 \text{pb}, \sigma_{pp\rightarrow \psi(2S) (\rightarrow \mu^{+} \mu^{-})} (2.0 <\eta_{\mu^{\pm}}< 4.5) = 7.8 \pm 1.3 \pm 1.0 \text{pb}, $ where the first uncertainties are statistical and the second are systematic. The measurements are found to be in good agreement with results from previous experiments and theoretical predictions. The $J/\psi$ photoproduction cross-section has been measured as a function of the photon-proton centre-of-mass energy. The results are consistent with measurements obtained at HERA and confirm a similar power law behaviour for the photoproduction cross-section.

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Number of tracks in the forward region for dimuon triggered events which in the backward region have (left) one or more tracks or (right) no tracks.

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Invariant mass distributions in the regions of (left) the $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} $ and (right) $\psi {(2S)} $ mass peaks for events with exactly two tracks, no photons and a dimuon with $ p_{\rm T} $ below 900$ {\mathrm{\,MeV\!/}c} $. The overall fits to the data are shown by the full curves while the dashed curves show the background contributions.

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Feynman diagrams displaying (a) exclusive $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} $ photoproduction and (b) inelastic $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} $ photoproduction where a small number of additional particles are produced due to gluon radiation and (c,d) proton dissociation.

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Transverse momentum distribution for exclusive $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} $ candidates with exactly two tracks and no photons. The points represent the data. The fit contains an exclusive signal component (short-dashed green curve) as estimated by SuperChic and an inelastic background component (long-dashed red curve) as estimated from data.

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(a) Transverse momentum distribution in $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} $ events with no backward tracks and no photons when there are three (black squares) and eight (red open circles) forward tracks. The results of fitting a Novosibirsk function to the three and eight forward track data are represented by the dashed black and the full red curves respectively. The values for (b) $P_{0}$, (c) $\sigma$ and (d) $\alpha$ of the $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} $ $ p_{\rm T} $ distribution are shown as a function of the number of forward tracks.

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Dependence of $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} $ photoproduction cross-section on the centre-of-mass energy of the photon-proton system. The blue (red) triangles represent the data from H1 (ZEUS) [3,4]. The black dots and squares are derived from the LHCb differential cross-section as a function of rapidity. The dashed and full lines are the power law dependences determined from the HERA and LHCb data, respectively. The uncertainty on the LHCb power law determination is shown by the shaded band.

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Animated gif made out of all figures.

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Tables and captions

Summary of selection requirements.

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Relative systematic uncertainties on the measurement.

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Comparison of cross-section times branching fraction measurements (pb) with theoretical predictions.

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Cross-section measurements (nb) as a function of $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} $ rapidity.

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Created on 09 December 2018.Citation count from INSPIRE on 18 December 2018.