Flavor models for $\bar{B} \to D^{(*)} \tau \bar{\nu}$

Abstract

The ratio of the measured $\bar B\to D^{(*)}\ell\bar{\nu}$ decay rates for $\ell = \tau$ vs. $e,\mu$ deviate from the Standard Model (SM) by about $4\sigma$. We show that the data are in tension with the SM, independent of form factor calculations, and we update the SM prediction for $\mathcal{B}(B\to X_c\tau\bar{\nu}) / \mathcal{B}(B\to X_c \ell\bar{\nu})$. We classify the operators that can accommodate the measured central values, as well as their UV completions. We identify models with leptoquark mediators that are minimally flavor violating in the quark sector, and are minimally flavor violating or $\tau$-aligned in the lepton sector. We explore experimental signatures of these scenarios, which are observable in the future at ATLAS/CMS, LHCb, or Belle II.