Flavor symmetry breaking in mixed-action QCD
  We study the phase structure of mixed-action QCD with two Wilson sea quarks and two chiral valence quarks, starting from the chiral lagrangian. A priori, the effective theory allows for a rich phase structure, including a phase with a condensate made of sea and valence quarks. Because this would lead to mass eigenstates that are admixtures of sea and valence fields, pure-sea correlation functions would depend on valence quark masses, in contradiction with the actual setup of mixed-action simulations. Using properties of the chiral Dirac operator, we prove that such a phase does not occur, and that this leads to bounds on low-energy constants. 
Oliver Baer (Humboldt U.), Maarten Golterman (SFSU), Yigal Shamir (Tel   Aviv U.)
hep-lat