$\mathrm{\mid }{V}_{cb}\mathrm{\mid }$ from the ${\bar{B}}^0\to D^{\ast +}{\mathrm{\ell }}^{-}\bar{\nu }$ zero-recoil form factor using $2+1+1$ flavour HISQ and NRQCD

We present the status of our ongoing calculation of the zero-recoil form factor for the semileptonic decay ${\bar{B}}^0\to D^{\ast +}{l}^{-}\bar{\nu }$ using lattice QCD with 2+1+1 flavours of highly improved staggered quarks in the sea (the MILC HISQ configurations) and using non-relativistic QCD for the bottom quark. We combine our result for $F(1)$ with the latest HFAG average of ${\eta }_{EW}F(1)\mathrm{\mid }{V}_{cb}\mathrm{\mid }$ to get a preliminary value for $\mathrm{\mid }{V}_{cb}\mathrm{\mid }$.