B-Meson Decay Constant from Unquenched Lattice QCD

We present determinations of the $B$-meson decay constant $f_B$ and of the ratio $f_{B_s}/f_B$ using the MILC Collaboration unquenched gauge configurations, which include three flavors of light sea quarks. The mass of one of the sea quarks is kept around the strange quark mass, and we explore a range in masses for the two lighter sea quarks down to $m_s/8$. The heavy $b$ quark is simulated using nonrelativistic QCD, and both the valence and sea light quarks are represented by the highly improved (AsqTad) staggered quark action. The good chiral properties of the latter action allow for a more accurate chiral extrapolation to physical up and down quarks than has been possible in the past. We find $f_B=216(9)(19)(4)(6)\mathrm{MeV}$ and $f_{B_s}/f_B=1.20(3)(1)$.