Phase transitions, θ behavior, and instantons in QCD and its holographic model

In the holographic model of QCD, θ dependence sharply changes at the point of confinement-deconfinement phase transition. In large N QCD such a change in θ behavior can be related to the breakdown of the instanton expansion at some critical temperature T_c. Associating this temperature with confinement-deconfinement phase transition leads to the description of the latter in terms of dissociation of instantons into the fractionally charged instanton quarks. To elucidate this picture, we introduce the nonvanishing chiral condensate in the deconfining phase and assume a specific Lagrangian for the η^' field in the confining phase. In the resulting picture the high-temperature phase of the theory consists of the dilute gas of instantons, while the low-temperature phase is described in terms of freely moving fractional instanton quarks.