2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/179169Applying asymptotic expansions at threshold, we compute the two-loop QCD correction to the short-distance coefficient that governs the leptonic decay $ψ\to l^+ l^-$ of a S-wave quarkonium state and discuss its impact on the relation between the quarkonium non-relativistic wave function at the origin and the quarkonium decay constant in full QCD.7 pages, LaTeX, 1 figure included via epsf.styHigh Energy Physics - Phenomenologytext