Calculation of $\bar{B}\to D^{\ast }\mathrm{\ell }\bar{\nu }$ form factor at zero recoil using the Oktay-Kronfeld action

We present the first preliminary results for the semileptonic form factor $h_{A_1}(w=1)\mathrm{/}{\rho }_{A_j}$ at zero recoil for the $\bar B \rightarrow D^\ast \ell \bar \nu$ decay using lattice QCD with four flavors of sea quarks. We use the HISQ staggered action for the light valence and sea quarks (the MILC HISQ configurations), and the Oktay-Kronfeld (OK) action for the heavy valence quarks.