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Recent experimental results in $B$ physics from Belle, BaBar and LHCb suggestnew 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 andspecifically 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 $\DeltaA_{FB} = A_{FB}(B\to D^{*} \mu\nu) - A_{FB} (B\to D^{*} e \nu)$ 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 andanalysis techniques in $B\to D^{*+}\ell^- \bar{\nu}$ decays. Here we describe aMonte Carlo Event-generator tool based on EVTGEN developed to allow simulationof the NP signatures in $B\to D^*\ell^- \nu$, which arise due to theinterference between the SM and NP amplitudes. As a demonstration of theproposed approach, we exhibit some examples of NP couplings that are consistentwith current data and could explain the $\Delta A_{FB}$ anomaly in $B\toD^*\ell^- \nu$ while remaining consistent with other constraints. We show thatthe $\Delta$-type observables such as $\Delta A_{FB}$ and $\Delta S_5$eliminate most QCD uncertainties from form factors and allow for cleanmeasurements of NP. We introduce correlated observables that improve thesensitivity to NP. We discuss prospects for improved observables sensitive toNP couplings with the expected 50 ab$^{-1}$ of Belle II data, which seems to beideally suited for this class of measurements.

A new tool to search for physics beyond the Standard Model in ${\bar B}\to D^{*+}\ell^- {\bar\nu}$