We report the results of the lattice simulation of the CPN−1 sigma model
on S1s(large) × S1τ(small). We take a sufficiently large ratio of the circumferences to approximate the model on R×S1. For periodic boundary condition imposed in the S1τ direction, we show that the expectation value of the Polyakov loop undergoes a deconfinement crossover as the compactified circumference is decreased, where the peak of the associated susceptibility gets sharper for larger N. For ZN twisted boundary condition, we find that, even at relatively high β (small circumference), the regular N-sided polygon-shaped distributions of Polyakov loop leads to small expectation values of Polyakov loop, which implies unbroken ZN symmetry if sufficient statistics and large volumes are adopted. We also argue the existence of fractional instantons and bions by investigating the dependence of the Polyakov loop on S1s direction, which causes transition between ZN vacua.
