2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/187081We present the coefficient of the chromomagnetic interaction operator, the only unknown coefficient in the Heavy Quark Effective Theory (HQET) lagrangian up to the $1/m$ level, with the two-loop accuracy by matching scattering amplitudes of an on-shell heavy quark in an external field in full QCD and HQET, and obtain the two-loop anomalous dimension of this operator in HQET.10 pages, LaTeX2e, 2 eps figures included. Revised discussion of (n_f-1)-flavour HQET includedHigh Energy Physics - Phenomenologytext