fBwith lattice NRQCD includingO(1/mQ2)corrections

We investigate higher order effects in the nonrelativistic expansion of lattice QCD (NRQCD) on the heavy-light meson decay constants and some other quantities in order to understand the truncation error of NRQCD. While our numerical results have large $O\left(a\right)\mathrm{}$ and $O\left(\alpha \right)$ errors due to the use of the Wilson light quark action and tree-level matching, we find that the truncation error of higher order relativistic corrections are adequately small around the mass of the $b$ quark. Simulations are carried out on a ${16}^3\times 32$ lattice with 120 quenched gauge configurations generated with the plaquette action at $\beta =5.8\mathrm{}$. Systematic errors and the limitation of NRQCD theory are discussed.