Flavor anomalies on the eve of the Run-2 verdict

Abstract

The [Formula: see text] measurement by LHCb suggests nonstandard Lepton-Universality Violation (LUV) to occur in [Formula: see text] decays with effects in muons rather than electrons. It is intriguing that a number of other measurements of [Formula: see text] transitions by LHCb and B-factories are consistent in magnitude and sign with the [Formula: see text] effect, and fit a coherent effective-theory picture. Further indications of nonstandard LUV are provided by the long-standing discrepancies in [Formula: see text] transitions via the ratios [Formula: see text] and [Formula: see text]. We review in detail the experimental situation and its rich outlook, the theoretical efforts — and their challenges — towards convincing dynamics beyond the effective-theory level, and discuss the many directions of further investigation that propagate from the current situation.