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

Abstract

The first observation of the decay ηc (2 S) → p p bar 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 (2 S) resonance is produced in the decay B+ → [ c c bar ]K+. The product of branching fractions normalised to that for the J / ψ intermediate state, R ηc (2 S), is measured to be R-eta c,(2s) equivalent to B(B+-> eta c (2S)K+) x B(eta c(2s)(2S) -> p<<(p)over bar> )/B(B+ -> J/psi k(+)) x B(J/psi -> p<<(p)over bar> ) = (1.58 +/- 0.33 +/- 0.09) x 10(-2), where the first uncertainty is statistical and the second systematic. No signals for the decays B+ -> X(3872)(-> p (p) over bar )K+ and B+ psi(3770)(-> p (p) over bar )K+ are seen, and the 95% confidence level upper limits on their relative branching ratios are found to be R-x(3872) < 0.25 x 10(-2) and R-psi(3770) < 0.10. In addition, the mass differences between the nc(1S) and the J/psi states, between the eta(c)(2S) and the eta(c)(2S) states, and the natural width of the psi(1S) are measured as M-J/psi - M-eta c(7s)= 110.2 +/- 0.5 +/- 0.9 MeV, M psi(2S) - M-eta c (2S) = 52.5 +/- 1.7 +/- 0.6 MeV, Gamma(eta c) (1s) = 34.0 +/- 1.9 +/- 1.3 MeV.