2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/206664The Bethe-Salpeter equation is related to a generalized quantum-mechanical Hamiltonian. Instability of the presumed vacuum, indicated by a tachyon, is related to a negative energy eigenstate of this Hamiltonian. The variational method shows that an arbitrarily weak long-range attraction leads to chiral symmetry breaking, except in the scale-invariant case when the instability occurs at a critical value of the coupling. In the case of short-range attraction, an upper bound for the critical coupling is obtained.10 pages, 2 figures; made minor changes, published versionHigh Energy Physics - TheorySuperconductivityHigh Energy Physics - Phenomenologytext