Observation of ηc (2 S ) → p p ̄ and search for X (3872) → p p ̄ decays

The first observation of the decay ηc (2 S ) → p p ̄ is reported using proton–proton sponding to an integrated luminosity of 3.0 fb−1 recorded by the LHCb experiment at centre-of-mass energies of 7 and 8 TeV. The ηc(2S) resonance is produced in the decay B+ →[cc ̄]K+. The product of branching fractions normalised to that for the J/ψ intermediate state, Rηc(2S), is measured to be Rηc(2S)≡ B ( B + → η c ( 2 S ) K + ) × B ( η c ( 2 S ) → p p ̄ ) / B(B+→J/ψK+)×B(J/ψ→pp ̄) =(1.58±0.33±0.09)×10 -2, where the first uncertainty is statistical and the second systematic. No signals for the decays B+ → X(3872)(→ pp ̄)K+ and B+ → ψ(3770)(→ pp ̄)K+ are seen, and the 95% confidence level upper limits on their relative branching ratios are found to be RX(3872) < 0.25 × 10−2 and Rψ(3770) < 0.10. In addition, the mass differences between the ηc (1 S ) and the J /ψ states, between the ηc (2 S ) and the ψ (2 S ) states, and the natural width of the ηc(1S) are measured as MJ/ψ −Mηc(1S) =110.2±0.5±0.9MeV, Mψ(2S) − Mηc (2S) = 52.5 ± 1.7 ± 0.6 MeV, ηc(1S) =34.0±1.9±1.3MeV