B-Meson Decay Constants from Improved Lattice Nonrelativistic QCD with Physical u,d,s, and c Quarks

We present the first lattice QCD calculation of the decay constants f(B) and f(Bs) with physical light quark masses. We use configurations generated by the MILC Collaboration including the effect of u, d, s, and c highly improved staggered quarks in the sea at three lattice spacings and with three u/d quark mass values going down to the physical value. We use improved nonrelativistic QCD (NRQCD) for the valence b quarks. Our results are f(B) = 0.186(4) GeV, f(Bs) = 0.224(4) GeV, f(Bs) / f(B) = 1.205(7), and M-Bs - M-B = 85(2) MeV, superseding earlier results with NRQCD b quarks. We discuss the implications of our results for the standard model rates for B-(s) ->mu(+)mu(-) and B ->tau nu