2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/45832The haaron gas description is reviewed for the QCD vacuum. The role of non-renormalizable operators is emphasised in the mechanism which generates the string tension. Additional examples are mentioned where certain non-renormalizable operators of the bare lagrangian turn out to be important at finite energy scale.Lecture delivered at the Enrico Fermi School of Physics, 37 pages, LATEXHigh Energy Physics - Phenomenologytext