Large $\theta$ angle in two-dimensional large $N$ ${\mathbb{CP}}^{N-1}$ model

Abstract

In confining large $N$ theories with a $\theta$ angle such as four-dimensional $\mathrm{SU}(N)$ pure Yang-Mills theory, there are multiple metastable vacua and it makes sense to consider the parameter region of ``large $\theta$ of order $N$'' despite the fact that $\theta$ is a $2\pi$-periodic parameter. We investigate this parameter region in the two-dimensional ${\mathbb{CP}}^{N-1}$ model by computing the partition function on $T^2$. When $\theta /N$ is of order $\mathcal{O}(0.1)$ or less, we get perfectly sensible results for the vacuum energies and decay rates of metastable vacua. However, when $\theta /N$ is of order $\mathcal{O}(1)$, we encounter a problem about saddle points that would give larger contributions to the partition function than the true vacuum. We discuss why it might not be straightforward to resolve this problem.

Comments: 35 pages, 4 figures; v2: references and explanations added, published version