Observation of ηc(2S) → p anti-p and Search for X(3872) → p anti-p Decays

The first observation of the decay ηc(2S)→ p anti-p 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+ → [c anti-c] K+. The product of branching fractions normalised to that for the J/ψ intermediate state, R_ηc(2S), is measured to be R_ηc(2S) ≡ (B(B+ → ηc(2S) K+) × B(ηc(2S) → p anti-p)) / (B(B+ → J/ψ K+) × B(J/ψ → p anti-p)) = (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)(→ p anti-p) K+ and B+ → ψ(3770) (→ p anti-p) K+ are seen, and the 95% confidence level upper limits on their relative branching ratios are found to be R_X(3872) < 0.25 × 10^−2 and R_ψ(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 − M_ηc(1S) = 110.2 ± 0.5 ± 0.9 MeV, M_ψ(2S) − M_ηc(2S) = 52.5 ± 1.7 ± 0.6 MeV, Γ_ηc(1S) = 34.0 ± 1.9 ± 1.3 MeV.