Neutral B Meson Mixing in Unquenched Lattice QCD

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We study Bd and Bₛ mixing in unquenched lattice QCD employing the MILC collaboration gauge configurations that include u, d, and s sea quarks based on the improved staggered quark (AsqTad) action and a highly improved gluon action. We implement the valence light quarks also with the AsqTad action and use the nonrelativistic NRQCD action for the valence b quark. We calculate hadronic matrix elements necessary for extracting CKM matrix elements from experimental measurements of mass differences ΔMd and ΔMₛ. We find ξ= fBₛ √BBₛ / fBd √BBd = 1.258(33), fBd √BBd = 216(15) MeV and fBₛ √BBₛ = 266(18) MeV. We also update previous results for decay constants and obtain fBd = 190(13) MeV, fBₛ = 231(15) MeV and fBₛ/fBd = 1.226(26). The new lattice results lead to updated values for the ratio of CKM matrix elements |Vtd|/|Vts| and for the Standard Model prediction for Br(Bₛ → μ+ μ-) with reduced errors. We determine |Vtd|/|Vts| = 0.214(1)(5) and Br(Bₛ → μ+ μ-) = 3.19(19) × 10⁻⁹.