Observation of η_c(2S)→pp¯ and search for X(3872)→pp¯ decays

The first observation of the decay ηc (2S) → pp¯ is reported using proton–proton collision data corresponding 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+ → ηc (2S)K +) × B(ηc (2S) → pp¯) B(B+ → J/ψ K +) × 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 (1S) and the J/ψ states, between the ηc (2S) and the ψ(2S) states, and the natural width of the ηc (1S) are measured as M J/ψ − Mηc (1S) = 110.2 ± 0.5 ± 0.9 MeV, Mψ(2S) − Mηc (2S) = 52.5 ± 1.7 ± 0.6 MeV, ηc (1S) = 34.0 ± 1.9 ± 1.3 MeV.