  Recent experimental results in $B$ physics from Belle, BaBar and LHCb suggest
new physics (NP) in the weak $b\to c$ charged-current and the $b\to s$
neutral-current processes. Here we focus on the charged-current case and
specifically on the decay modes $B\to D^{*+}\ell^- \bar{\nu}$ with $\ell = e,
\mu,$ and $\tau$. The world averages of the ratios $R_D$ and $R_D^{*}$
currently differ from the Standard Model (SM) by $3.4\sigma$ while $\Delta
A_{FB} = A_{FB}(B\to D^{*} \mu\nu) - A_{FB} (B\to D^{*} e \nu)$ is found to be
$4.1\sigma$ away from the SM prediction in an analysis of 2019 Belle data.
These intriguing results suggest an urgent need for improved simulation and
analysis techniques in $B\to D^{*+}\ell^- \bar{\nu}$ decays. Here we describe a
Monte Carlo Event-generator tool based on EVTGEN developed to allow simulation
of the NP signatures in $B\to D^*\ell^- \nu$, which arise due to the
interference between the SM and NP amplitudes. As a demonstration of the
proposed approach, we exhibit some examples of NP couplings that are consistent
with current data and could explain the $\Delta A_{FB}$ anomaly in $B\to
D^*\ell^- \nu$ while remaining consistent with other constraints. We show that
the $\Delta$-type observables such as $\Delta A_{FB}$ and $\Delta S_5$
eliminate most QCD uncertainties from form factors and allow for clean
measurements of NP. We introduce correlated observables that improve the
sensitivity to NP. We discuss prospects for improved observables sensitive to
NP couplings with the expected 50 ab$^{-1}$ of Belle II data, which seems to be
ideally suited for this class of measurements.
