Large $θ$ angle in two-dimensional large $N$ $\mathbb{CP}^{N-1}$ model
In confining large $N$ theories with a $θ$ 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 $θ$ of order $N$'' despite the fact that $θ$ is a $2π$-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 $θ/N$ 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 $θ/N$ 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.