Relativistic structure of heavy quarkonia. Application to their leptonic decays

We investigate the relativistic structure of J=1, L=0 heavy quarkonia in the framework of a covariant formulation of light-front dynamics. This formalism enables us to treat in a very transparent way off-energy shell effects as well as relativistic kinematical effects coming from the finite momentum between the quark and antiquark. We apply our formalism to the leptonic decay width of J=1 charmonium and bottomonium S-states. Relativistic corrections are found to be very large in the charm sector (up to about 60 % reduction for ψ′ leptonic decay width) and smaller in the bottomonium sector, but in any case not negligible.