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 cc¯-states with quantum numbers JPC=0−+ (ηc), 1−− (J/ψ), and 1+− (hc) by using lattice QCD and QCD sum rules. We consider the coupling of J/ψ to both the vector and tensor currents. Lattice QCD results are obtained from the unquenched (Nf=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 ηc→γγ decay, and to comment on the factorization in B→Xcc¯K decays, with Xcc¯ being either ηc or J/ψ