Narrow Results

In nonrelativistic systems, when there is spontaneous symmetry breaking, the number of Nambu–Goldstone bosons $(n_{\mathrm{\text{NG}}})$ are not necessarily equal to the number of broken generators $(n_{\mathrm{\text{BG}}})$. Here we use the method of operators for analyzing the necessary conditions in order to obtain the correct dispersion relation for the Nambu–Goldstone bosons.

We explain the origin of the mass for the Nambu–Goldstone bosons when there is a chemical potential in the action which explicitly breaks the symmetry. The method is based on the number of independent histories for the interaction of the pair of Nambu–Goldstone bosons with the degenerate vacuum (triangle relations). The analysis suggests that under some circumstances, pairs of massive Nambu–Goldstone bosons can become a single degree of freedom with an effective mass defined by the superposition of the individual masses of each boson. Possible mass oscillations for the Nambu–Goldstone bosons are discussed.

We investigate thermodynamics of hadrons using the Gaussian functional method (GFM) at finite temperature. Since the interaction among mesons is very large, we take into account fluctuations of mesons around their mean field values using the GFM. We obtain the ground state energy by solving the Schrödinger equation. The meson masses are obtained using the energy minimization condition. The resulting mass of pion is not zero even in the spontaneous chiral symmetry broken phase due to the non-perturbative effect. We consider then the bound state of mesons using the Bethe-Salpeter equation and show that the Nambu-Goldstone theorem is recovered. We investigate further the behavior of the meson masses and the mean filed value as functions of temperature for the cases of chiral limit and explicit chiral symmetry breaking.