Measurement of the ηc(1S) production cross-section in proton–proton collisions via the decay ηc(1S)→pp¯

The production of the ηc(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 pT>6.5GeV/c . The cross-section for prompt production of ηc(1S) mesons relative to the prompt J/ψ cross-section is measured, for the first time, to be σηc(1S)/σJ/ψ=1.74±0.29±0.28±0.18B at a centre-of-mass energy s=7TeV using data corresponding to an integrated luminosity of 0.7 fb -1 , and σηc(1S)/σJ/ψ=1.60±0.29±0.25±0.17B at s=8TeV using 2.0 fb -1 . The uncertainties quoted are, in order, statistical, systematic, and that on the ratio of branching fractions of the ηc(1S) and J/ψ decays to the pp¯ final state. In addition, the inclusive branching fraction of b -hadron decays into ηc(1S) mesons is measured, for the first time, to be B(b→ηcX)=(4.88±0.64±0.29±0.67B)×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 ηc(1S) meson masses is determined to be 114.7±1.5±0.1MeV/c2 .