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Double poles in Lattice QCD with mixed actions

We consider effects resulting from the use of different discretizations for the valence and the sea quarks, considering Wilson and/or Ginsparg--Wilson fermions. We assume that such effects appear through scaling violations that can be studied using effective-lagrangian techniques. We show that a double pole is present in flavor-neutral Goldstone meson propagators, even if the flavor non-diagonal Goldstone mesons made out of valence or sea quark have equal masses. We then consider some observables known to be anomalously sensitive to the presence of a double pole. We find that the double-pole enhanced scaling violations may turn out to be rather small in practice.