${\mathbb{Z}}_N$ structure of deconfinement vacuum in SU($N$) Yang-Mills theory: emergence of Nambu-Goldstone mode in large-$N$ limit

Using the Polyakov-loop effective action, we investigate the structure of spontaneously broken ${\mathbb{Z}}_N$ symmetry in the deconfinement vacuum in the SU($N$) Yang-Mills theory with finite $N$. First, we examine the Polyakov-loop fluctuation around a ${\mathbb{Z}}_N$-broken vacuum and calculate the spatial correlation of its phase variable. We show that the phase variable of the Polyakov loop becomes a Nambu-Goldstone mode in the large-$N$ limit. Second, we estimate the global vacuum-to-vacuum transition rate in a finite-volume domain of the quark-gluon plasma. Based on our estimation, we state that some threshold volume exists, a domain larger than which is stable, and vice versa. Identifying the threshold as the lower bound of a stable center domain volume, we find the typical volume scale of center domains.