Phase structure with nonzero $\Theta_{\rm QCD}$ and twisted mass fermions

Abstract

We determine the phase diagram and chiral condensate for lattice QCD with two flavors of twisted-mass fermions in the presence of nondegenerate up and down quarks, discretization errors and a nonzero value of $\Theta_{\rm QCD}$. Although such a theory has a complex action and cannot, at present, be simulated, the results are needed to understand how to tune to maximal twist in the presence of electromagnetism, a topic discussed in a companion paper. We find that, in general, the only phase structure is a first-order transition of finite length. Pion masses are nonvanishing throughout the phase plane except at the endpoints of the first-order line. Only for extremal values of the twist angle and $\Theta_{\rm QCD}$ ($\omega=0$ or $\pi/2$ and $\Theta_{\rm QCD}=0$ or $\pi$) are there second-order transitions.