B-meson mixing from full lattice QCD with physical u, d, s and c quarks
  We present the first lattice QCD calculation of the $B_s$ and $B_d$ mixing parameters with physical light quark masses. We use MILC gluon field configurations that include $u$, $d$, $s$ and $c$ sea quarks at 3 values of the lattice spacing and with 3 values of the $u/d$ quark mass going down to the physical value. We use improved NRQCD for the valence $b$ quarks. Preliminary results show significant improvements over earlier values. 
R. J. Dowdall, C. T. H. Davies, R. R. Horgan, G. P. Lepage, C. J.   Monahan and J. Shigemitsu
hep-lat