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The phenomenology of leptonic decays of quarkonia holds many interestingfeatures: for instance, it can establish constraints on scenarios beyond theStandard Model with the Higgs sector enriched by a light CP-odd state. In thefollowing paper, we report on a two-flavor lattice QCD study of the $\eta_c$and $J/\psi$ decay constants, $f_{\eta_c}$ and $f_{J\psi}$. We also examinesome properties of the first radial excitation $\eta_c(2S)$ and $\psi(2S)$.

Some hadronic parameters of charmonia in $\boldsymbol{N_{\text{f}}=2}$ lattice QCD