QCD at theta similar to pi - art. no. 114021.

Taking into account the terms similar to m(2) in the effective chiral Lagra ngian, 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, the energy barrier between two vacua appears already in the leading o rder in mass. This corresponds to the first order phase transition at theta = pi. The surface energy density of the domain wall separating two differe nt 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. [S0556-2821(99)00 711-0].