Measurement of the eta(c)(1S) production cross-section in p p collisions at root s=13TeV

Using a data sample corresponding to an integrated luminosity of 2.0 fb-1, collected by the LHCb experiment, the production of the \u3b7c(1 S) state in proton\u2013proton collisions at a centre-of-mass energy of s=13TeV is studied in the rapidity range 2.0 < y< 4.5 and in the transverse momentum range 6.5<14.0GeV. The cross-section for prompt production of \u3b7c(1 S) mesons relative to that of the J/ \u3c8 meson is measured using the pp\uaf decay mode and is found to be \u3c3\u3b7c(1S)/\u3c3J/\u3c8=1.69\ub10.15\ub10.10\ub10.18. The quoted uncertainties are, in order, statistical, systematic and due to uncertainties on the branching fractions of the J/\u3c8\u2192pp\uaf and \u3b7c\u2192pp\uaf decays. The prompt \u3b7c(1 S) production cross-section is determined to be \u3c3\u3b7c(1S)=1.26\ub10.11\ub10.08\ub10.14\u3bcb, where the last uncertainty includes that on the J/ \u3c8 meson cross-section. The ratio of the branching fractions of b-hadron decays to the \u3b7c(1 S) and J/ \u3c8 states is measured to be Bb\u2192\u3b7cX/Bb\u2192J/\u3c8X=0.48\ub10.03\ub10.03\ub10.05, where the last uncertainty is due to those on the branching fractions of the J/\u3c8\u2192pp\uaf and \u3b7c\u2192pp\uaf decays. The difference between the J/ \u3c8 and \u3b7c(1 S) masses is also determined to be 113.0\ub10.7\ub10.1MeV, which is the most precise single measurement of this quantity to date