2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/16804We point out that the quasiparticle spectrum of the Landau Fermi liquid theory has an extra $Z_2$ symmetry, local in momentum space, which is not generic to the Hamiltonian with interactions. Thus the Fermi liquid is in this sense a (quantum) zero-temperature critical point.submitted to a volume honoring Quin LuttingerStrongly Correlated Electronstext