QCD atθ∼π

Taking into account the terms $\sim m^2$ in the effective chiral Lagrangian, we show that, at $\theta =\pi$, the theory with two light quarks of equal mass involves two degenerate vacuum states separated by a barrier. For $N_f=3\mathrm{}$, the energy barrier between two vacua appears already in the leading order in mass. This corresponds to the first order phase transition at $\theta =\pi$. The surface energy density of the domain wall separating two different vacua is calculated. In the immediate vicinity of the point $\theta =\pi$, two minima of the potential still exist, but one of them becomes metastable. The probability of the false vacuum decay is estimated.