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

Exclusive $J/ψ$ and $ψ(2S)$ 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 $ σ_{pp\rightarrow J/ψ(\rightarrow μ^{+} μ^{-})} (2.0 <η_{μ^{\pm}}< 4.5) = 307 \pm 21 \pm 36 \text{pb}, σ_{pp\rightarrow ψ(2S) (\rightarrow μ^{+} μ^{-})} (2.0 <η_{μ^{\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 $J/ψ$ 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.