View the article online for updates and enhancements. Related contentUpdated measurements of exclusive J/ \psi and \psi (2S) production crosssections in pp collisions at \sqrt s= 7 TeV R Aaij, B Adeva, M Adinolfi et al. Abstract 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→J/ψ (→μ + μ − ) (2.0 < η μ ± < 4.5) = 307 ± 21 ± 36 pb, σ pp→ψ(2S)(→μ + μ − ) (2.0 < η μ ± < 4.5) = 7.8 ± 1.3 ± 1.0 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-ofmass energy. The results are consistent with measurements obtained at HERA and confirm a similar power law behaviour for the photoproduction crosssection.

The decays χ_{c1}→J/ψμ^{+}μ^{-} and χ_{c2}→J/ψμ^{+}μ^{-} are observed and used to study the resonance parameters of the χ_{c1} and χ_{c2} mesons. The masses of these states are measured to be m(χ_{c1})=3510.71±0.04(stat)±0.09(syst)  MeV and m(χ_{c2})=3556.10±0.06(stat)±0.11(syst)  MeV, where the knowledge of the momentum scale for charged particles dominates the systematic uncertainty. The momentum-scale uncertainties largely cancel in the mass difference m(χ_{c2})-m(χ_{c1})=45.39±0.07(stat)±0.03(syst)  MeV. The natural width of the χ_{c2} meson is measured to be Γ(χ_{c2})=2.10±0.20(stat)±0.02(syst)  MeV. These results are in good agreement with and have comparable precision to the current world averages.