NeutralBmeson mixing in unquenched lattice QCD

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 Cabibbo-Kobayashi-Maskawa matrix elements from experimental measurements of mass differences ΔMd and ΔMₛ. We find ξ≡f_Bₛ√^B_Bₛ/f_Bd√^B_Bd=1.258(33), f_Bd√^B_Bd=216(15) MeV, and f_Bₛ√^B_Bₛ=266(18) MeV. We also update previous results for decay constants and obtain f_Bd=190(13) MeV, f_Bₛ=231(15) MeV, and f_Bₛ/f_Bd=1.226(26). The new lattice results lead to updated values for the ratio of Cabibbo-Kobayashi-Maskawa 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.