Measurement of the ηc(1S) production cross-section in proton–proton collisions via the decay ηc(1S) → pp: see text] production cross-section in proton-proton collisions via the decay [Formula: see text]

The production of the η _c(1S) state in proton-proton collisions is probed via its decay to the pp final state with the LHCb detector, in the rapidity range 2.0 < y < 4.5 and in the meson transverse-momentum range p _T > 6.5 GeV/c. The cross-section for prompt production of η _c;(1S) mesons relative to the prompt J/ψ cross-section is measured, for the first time, to be σ _ηc(1S)/σ _J/ψ = 1.74±0.29±0.28±0. 18 _B at a centre-of-mass energy (Formula presented.) using data corresponding to an integrated luminosity of 0.7 fb ^-1, and σ _ηc(1S)/σJ/ψ = 1.60±0.29±0.25±0.17 _B at (Formula presented.) using 2.0 fb ^-1. The uncertainties quoted are, in order, statistical, systematic, and that on the ratio of branching fractions of the η _c(1S) and J/ψ decays to the pp final state. In addition, the inclusive branching fraction of b-hadron decays into η _c(1S) mesons is measured, for the first time, to be B(b→η _cX)=(4.88±0.64±0.29±0.67 _B)× ^10-3, where the third uncertainty includes also the uncertainty on the J/ψ inclusive branching fraction from b-hadron decays. The difference between the J/ψ and η _c(1S) meson masses is determined to be 114.7±1.5±0.1MeV/c ².