Implications for the $\Delta A_{FB}$ anomaly in ${\bar B}^0\to D^{*+}\ell^- {\bar\nu}$ using a new Monte Carlo Event Generator

Abstract

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 $\bar{B}^0\to D^{*+}\ell^- \bar{\nu}$ with $\ell = e$ and $\mu$. The world averages of the ratios $R_D$ and $R_D^{*}$ currently differ from the Standard Model (SM) predictions by $3.4\sigma$ while recently a new anomaly has been observed in the forward-backward asymmetry measurement, $A_{FB}$, in $ \bar{B}^0\to D^{*+}\mu^- \bar{\nu}$ decay. It is found that $\Delta A_{FB} = A_{FB}(B\to D^{*} \mu\nu) - A_{FB} (B\to D^{*} e \nu)$ is around $4.1\sigma$ away from the SM prediction in an analysis of 2019 Belle data. In this work we explore possible solutions to the $\Delta A_{FB}$ anomaly and point out correlated NP signals in other angular observables. These correlations between angular observables must be present in the case of beyond the Standard Model physics. We stress the importance of $\Delta$ type observables that are obtained by taking the difference of the observable for the muon and the electron mode. These quantities cancel form factor uncertainties in the SM and allow for clean tests of NP. These intriguing results also suggest an urgent need for improved simulation and analysis techniques in $\bar{B}^0\to D^{*+}\ell^- \bar{\nu}$ decays. Here we also describe a new Monte Carlo Event-generator tool based on EVTGEN that we developed to allow simulation of the NP signatures in $\bar{B}^0\to D^{*+}\ell^- \nu$, which arise due to the interference between the SM and NP amplitudes. We then discuss prospects for improved observables sensitive to NP couplings with 1, 5, 50, and 250 ab$^{-1}$ of Belle II data, which seem to be ideally suited for this class of measurements.