Two-Loop Corrections to the Leptonic Decays of Quarkonium

Applying asymptotic expansions at threshold, we compute the two-loop quantum chromodynamics (QCD) correction to the short-distance coefficient that governs the leptonic decay ψ→l⁺l^- of an S-wave quarkonium state and discuss its impact on the relation between the quarkonium nonrelativistic wave function at the origin and the quarkonium decay constant in full QCD.