Theta vacuum effects on QCD phase diagram

Theta vacuum effects on the QCD phase structure in the \mu-T plane are studied by using the Polyakov-loop extended Nambu-Jona-Lasinio model and its extension, where \mu is the quark chemical potential and T is temperature, respectively. As the parameter \theta of the theta vacuum increases, the chiral transition becomes stronger. For large \theta, it eventually becomes first order even at zero \mu.