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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., 14 pages, 9 figures

Using SU(3) lattice QCD, we investigate role of spatial gluons for hadron masses in the Coulomb gauge, considering the relation between QCD and the quark model. From the Coulomb-gauge configurations at the quenched level on a $16^3 \times 32$ lattice at $\beta$ = 6.0, we consider the $\vec{A} = 0$ projection, where all the spatial gluon fields are set to zero. In this projection, the inter-quark potential is unchanged. We investigate light hadron masses and find that nucleon and delta baryon masses are almost degenerate. This result suggests that the N-$\Delta$ mass difference arises from the color-magnetic interactions, which is consistent with the quark model picture. Next, as a generalization of this projection, we expand spatial gluon fields in terms of Faddeev-Popov eigenmodes and leave only some partial components. We find that the ${\rm N}-\Delta$ and $0^{++}-2^{++}$ glueball mass splittings are almost reproduced only with 1 \% low-lying components. This suggests that low-lying color-magnetic interaction leads to the hadron mass splitting., Comment: 8 pages, 5 figures, contribution to the proceedings of the 15th Quark Confinement and the Hadron Spectrum conference (ConfXV), 1st-6th August 2022, Stavanger, Norway

Aiming at the relation between QCD and the quark model, we consider projections of gauge configurations generated in quenched lattice QCD simulations in the Coulomb gauge on a $16^3 \times 32, \beta = 6.0$ lattice. First, we focus on a fact that the static quark-antiquark potential is independent of spatial gauge fields. We explicitly confirm this by performing $\vec{A} = 0$ projection, where spatial gauge fields are all set to zero. We also apply the $\vec{A} = 0$ projection to light hadron masses and find that nucleon and delta baryon masses are almost degenerate, suggesting vanishing of the color-magnetic interactions. After considering the physical meaning of the $\vec{A} = 0$ projection, we next propose a generalized projection, where spatial gauge fields are expanded in terms of Faddeev-Popov eigenmodes and only some eigenmodes are left. We apply the proposed projection to light hadron and glueball masses and find that the $N-\Delta$ and $0^{++}-2^{++}$ mass splittings become evident when projected with more than 33 (0.10 \%) low-lying eigenmodes, suggesting emergence of the color-magnetic interactions. We also find that the original hadron masses are approximately reproduced with just 328 (1.00 \%) low-lying eigenmodes. These findings indicate an important role of low-lying eigenmodes on hadron masses and would be useful in clarifying the relation between QCD and the quark model., Comment: 11 pages, 9 figures, and 2 tables

Proceedings of the 14th International Conference on Meson-Nucleon Physics and the Structure of the Nucleon (MENU2016)