Spontaneous breakdown of γ5 and gauge symmetry at finite temperature

Within the context of the photon-neutrino weak interaction, the temperature dependence is studied of the composite character of the photon and the critical temperature above which the particle masses vanish. The result obtained implies the existence of two temperatures: one below which gauge symmetry breaks down and photon attains an effective mass, and another above which γ5-symmetry may be restored. These may have some relevance in the study of the early Universe and also in the theories of low-temperature physics. (author)