  We present the results of our lattice QCD study of the hadronic matrix
elements relevant to the physical radiative J/psi -> eta_c gamma and hc ->
eta_c gamma decays. From computations with Nf=2 dynamical quark in twisted mass
QCD at four lattice spacings, we were able to take the continuum limit and
obtained Gamma(J/psi -> eta_c gamma) = 2.64(11) keV and Gamma(hc -> eta_c
gamma) = 0.72(5) MeV. We also computed the the hyperfine splitting and found
that it does not depend from the sea quark mass and we obtain Delta = m_J/psi -
m_eta_c = 112(4) MeV.

High Energy Physics - Lattice