arXiv : The Forward-Backward Asymmetry in $B\to D^{*}\ell u$: One more hint for Scalar Leptoquarks?
Alexandre Carvunis, Andreas Crivellin, Diego Guadagnoli, Shireen Gangal · arXiv (Cornell University) · 2021
In recent years, intriguing hints for the violation of lepton flavour universality have accumulated. In particular, deviations from the Standard-Model (SM) predictions in $B\to D^{(*)}\tau u/B\to D^{(*)}\ell u$, in the anomalous magnetic moment of the muon and {in}$b\to s\ell^+\ell^-$ data were observed with a significance of $\!>3\,\sigma$, $>\!4\,\sigma$ and $>\!5\,\sigma$, respectively. Furthermore, in a recent re-analysis of 2018 Belle data, it was found that the forward-backward asymmetry of $\bar B \to D^{*}\mu\bar u$ vs $\bar B\to D^{*}e\bar u$ disagrees with the SM prediction by $\approx\!\!4\,\sigma$ which would be an additional sign of lepton flavour universality violation. Since one naturally expects muon-related new effects to also emerge at some point in $b \to c\mu u$ decays, the above putative deviation might share a common origin with the other flavour anomalies. We show that a tensor operator is necessary to significantly improve the global fit w.r.t. the SM, which can only be induced (at tree-level in a renormalizable model) by a scalar leptoquark. Interestingly, among the two possible representations, the $SU(2)_L$-singlet $S_1$ and the doublet $S_2$, which can both also account for the anomalous magnetic moment of the muon, only $S_1$ can provide a good fit as it naturally gives rise to the scenario $C_{VL}, C_{SL}=-4 C_T$. While the constraints from (differences of) other angular observables prefer a smaller value $\Delta A_{\rm FB}$, this scenario is significantly preferred ($\approx 3 \sigma$) over the SM hypothesis and compatible with constraints such as $B\to K^* u u$ and electroweak precision bounds.