Semitauonic $b$-hadron decays: A lepton flavor universality laboratory

TL;DR

This review comprehensively surveys experimental and theoretical aspects of lepton flavor universality violation in semitauonic $b$-hadron decays, highlighting future prospects for precision measurements and implications for new physics.

Contribution

It provides a detailed overview of current methodologies, theoretical predictions, and discusses how different data treatments affect the interpretation of LFUV anomalies.

Findings

Current $3\sigma$ tension with the SM can vary within $1\sigma$ based on uncertainty correlations.

Reassessment of $D^{**}\tau\nu$ contributions may influence the observed anomalies.

Future measurements could achieve percent-level precision to test SM predictions.

Abstract

The study of lepton flavor universality violation (LFUV) in semitauonic $b$-hadron decays has become increasingly important in light of longstanding anomalies in their measured branching fractions, and the very large datasets anticipated from the LHC and Belle II. In this review, we undertake a comprehensive survey of the experimental environments and methodologies for semitauonic LFUV measurements at the $B$-factories and LHCb, along with a concise overview of the theoretical foundations and predictions for a wide range of semileptonic decay observables. We proceed to examine the future prospects to control systematic uncertainties down to the percent level, matching the precision of Standard Model (SM) predictions. Furthermore, we discuss new perspectives and caveats on combinations of the LFUV data and revisit the world averages for the ${\cal R}(D^{(*)})$ ratios. Here we demonstrate that different treatments for the correlations of uncertainties from $D^{**}$ excited states can vary the current $3\sigma$ tension with the SM within a $1\sigma$ range. Prior experimental overestimates of $D^{**}\tau\nu$ contributions may further exacerbate this. The precision of future measurements is also estimated; their power to exploit full differential information, and solutions to the inherent difficulties in self-consistent new physics interpretations of LFUV observables, are briefly explored.