String-like phase in Yang-Mills theory

A dynamical mechanism is discussed leading to the phase where the string of the color flux tube is formed in Yang-Mills theory. The pairing force between massless gauge fields in the color singlet channel forms a tachyon or a Cooper state and leads to the instability of the normal vacuum. A stable vacuum is required to remain massless after the condensation of pairs. Then the stability of the new vacuum leads to the condition on the 't Hooft index α which is equivalent to the condition of the existence of the string solutions. (Auth.)