Lepton Flavor Universality Tests in Semileptonic b→c Decays

Semileptonic decays of b- to c-hadrons provide an exciting environment to probe new physics and currently present some of the most compelling anomalies in the field of flavor physics. Measurements of the lepton flavor universality ratios $\mathcal{R}\left(D^{*}\right)$, comparing branching fractions with $\tau$ and $\mu$ leptons, show a discrepancy of over 3σ with respect to the Standard Model, and suggest that the coupling to $\tau$ leptons is stronger than predicted. Measurements of angular distributions as well as polarization in b- to c-hadron decays provide additional sensitivity to new physics. This review article offers an overview of the theory of semileptonic b- to c-hadron decays, presents the experiments and experimental techniques used to perform measurements of these decays, and summarizes the latest experimental results with their implications.