Relativistic structure of heavy quarkonia. Application to their leptonic decays

We investigate the relativistic structure of J = 1, L = 0 heavy quarkonia i n the framework of a covariant formulation of light-front dynamics. This fo rmalism enables us to treat in a very transparent way off-energy shell effe cts as well as relativistic kinematical effects coming from the finite mome ntum between the quark and antiquark. We apply our formalism to the leptoni c decay width of J = 1 charmonium and bottomonium S-states. Relativistic co rrections are found to be very large in the charm sector (up to about 60 % reduction for psi' leptonic decay width) and smaller in the bottomonium sec tor; but in any case not negligible. (C) 2000 Published by Elsevier Science B.V. All rights reserved.