QCD at θ\sim π

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Abstract
Taking into account the quadratic in mass terms in the effective chiral lagrangian, we show that, at \theta \sim \pi, the theory with 2 light quarks of equal mass involves two degenerate vacuum states separated by a barrier. For three flavors, 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 phase transition point, two minima of the potential still exist, but one of them becomes metastable. The probability of the false vacuum decay is estimated.

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