Measurement of the ηc(1S) Production Cross-Section in pp Collisions at √s = 13 TeV

Using a data sample corresponding to an integrated luminosity of 2.0 fb^−1, collected by the LHCb experiment, the production of the ηc(1S) state in proton–proton collisions at a centre-of-mass energy of √s = 13 TeV is studied in the rapidity range 2.0 < y < 4.5 and in the transverse momentum range 6.5 < p_T < 14.0 GeV. The cross-section for prompt production of ηc(1S) mesons relative to that of the J/ψ meson is measured using the p anti-p decay mode and is found to be σ_ηc(1S) / σ_J/ψ = 1.69 ± 0.15 ± 0.10 ± 0.18. The quoted uncertainties are, in order, statistical, systematic and due to uncertainties on the branching fractions of the J/ψ → p anti-p and η_c → p anti-p decays. The prompt ηc(1S) production cross-section is determined to be σ_ηc(1S) = 1.26 ± 0.11 ± 0.08 ± 0.14 μb , where the last uncertainty includes that on the J/ψ meson cross-section. The ratio of the branching fractions of b-hadron decays to the ηc(1S) and J/ψ states is measured to be B_{b → ηc X} / B_{b → J/ψ X} = 0.48 ± 0.03 ± 0.03 ± 0.05 , where the last uncertainty is due to those on the branching fractions of the J/ψ → p anti-p and ηc → p anti-p decays. The difference between the J/ψ and ηc(1S) masses is also determined to be 113.0 ± 0.7 ± 0.1 MeV, which is the most precise single measurement of this quantity to date.