Effects of staggered fermions and mixed actions on the scalar correlator

We provide the analytic predictions for the flavor nonsinglet scalar correlator, which will enable determination of the scalar meson mass from the lattice scalar correlator. We consider simulations with 2 + 1 staggered sea quarks and staggered or chiral valence quarks. At small u/d masses the correlator is dominated by the bubble contribution, which is the intermediate state with two pseudoscalar mesons. We determine the bubble contribution within staggered and mixed chiral perturbation theory. Its effective mass is smaller than the mass of pi eta, which is the lightest intermediate state in proper 2 + 1 QCD. The unphysical effective mass is a consequence of the taste breaking that makes possible the intermediate state with mass 2M(pi). We find that the scalar correlator can be negative in the simulations with mixed quark actions if the sea- and valence-quark masses are tuned by matching the pion masses M-val,M-val=M-pi 5.