Heavy-heavy current improvement for calculation of $\bar{B}\to D^{(\ast )}\mathrm{\ell }\bar{\nu }$ semi-leptonic form factors using the Oktay-Kronfeld action

Lattice calculations of the form factors for $\bar{B}\to D^{()}\ell\bar{\nu}$decayscanbeusedtoextracttheCKMmatrixelement$|V_{cb}|$. The Oktay-Kronfeld action is a highly improved version of the Fermilab action, which systematically reduces heavy quark discretization effects through $\mathcal{O}({\lambda }^3)$ in HQET power counting, for heavy-light meson quantities. To calculate $\bar{B}\rightarrow D^{()}\ell \bar{\nu}$ semi-leptonic form factors using Oktay-Kronfeld heavy quarks, we need to improve the heavy quark currents to the same level. We report our progress in calculating the improvement coefficients for currents composed of bottom and charm quarks. Our results presented in this paper are preliminary.