Lattice QCD and QCD sum rule determination of the decay constants of ηc, J/ψ and hc, states

We compute the decay constants of the lowest c (c) over bar -states with quantum numbers J(PC) = 0(-+) (eta(c)), 1(--)(J/psi), and 1(+-) (h(c)) by using lattice QCD and QCD sum rules. We consider the coupling of J/psi to both the vector and tensor currents. Lattice QCD results are obtained from the unquenched (N-f = 2) simulations using twisted mass QCD at four lattice spacings, allowing us to take the continuum limit. On the QCD sum rule side we use the moment sum rules. The results are then used to discuss the rate of eta(c) -> gamma gamma decay, and to comment on the factorization in B -> Xc (c) over barK decays, with X-c (c) over bar being either eta(c) or J/psi. (C) 2014 The Authors. Published by Elsevier B.V.