Phase Transitions, theta Behavior and Instantons in QCD and its Holographic Model

Understand

In the holographic model of QCD, theta dependence sharply changes at the point of confinement deconfinement phase transition.

• In large N QCD such a change in theta 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 eta' field in the confining phase.