Lattice QCD estimate of the η c(2S) → J/ψγ decay rate

Abstract

We compute the hadronic matrix element relevant to the physical radiative decay ηc(2S) → J/ψγ by means of lattice QCD. We use the (maximally) twisted mass QCD action with Nf = 2 light dynamical quarks and from the computations made at four lattice spacings we were able to take the continuum limit. The value of the mass ratio mηc(2S)/mηc(1S) we obtain is consistent with the experimental value, and our prediction for the form factor is Vηc(2S)→J/ψΥ (0) ≡ V12(0) = 0.32(6)(2), leading to Γ(ηc(2S) → J/ψγ) = (15.7 ± 5.7) keV, which is much larger than Γ(ψ(2S) → ηcγ) and within reach of modern experiments.