High-precision lattice calculation of the decay constants $f_B$ and $f_{B_s}$

We present preliminary, high-precision results for the hadronic decay constants of the $B$ and the $B_s$ mesons from lattice QCD simulations using a highly improved quark formulation for both heavy and light valence quarks. Calculations are carried out with several heavy valence-quark masses on lattice ensembles with 2+1+1 flavors of HISQ sea quarks at five lattice spacings and three light sea quark mass ratios $m_{ud}/m_s$, including approximately physical sea quark masses. This range of parameters provides excellent control of the continuum limit and of heavy-quark discretization errors.