Review of Lepton Universality tests in B decays (1809.06229v2)

Abstract: Several measurements of tree- and loop-level b-hadron decays performed in the recent years hint at a possible violation of Lepton Universality. This article presents an experimental and theoretical overview of the current status of the field.

• The paper reviews lepton universality tests in semileptonic B decays by analyzing both theoretical formulations and experimental deviations from Standard Model predictions.
• It employs effective field theory to dissect short- and long-distance effects, with detailed assessments of branching fraction ratios such as R_K, R_K*, R_D, and R_D*.
• Future experiments at Belle-II and LHCb are anticipated to improve measurements, potentially uncovering evidence for physics beyond the Standard Model.

Analyzing Lepton Universality in Semileptonic $\mathbf{B}$ Decays

The paper "Review of Lepton Universality Tests in $\mathbf{B}$ Decays" provides a comprehensive analysis of the status and results related to lepton universality (LU) tests in semileptonic $b$-quark hadron decays. Focusing on exploring the differences between the different families of leptons within the context of the Standard Model (SM), the authors scrutinize both theoretical and experimental aspects, emphasizing results from the LHCb experiment at the Large Hadron Collider (LHC) and past experiments like BaBar and Belle.

Theoretical Framework and Significance

The SM predicts the equality of electroweak couplings for all lepton species, a principle termed lepton universality (LU). This paper lays out the theoretical formulation of LU within the SM, emphasizing the importance of effective field theories in separating short-distance contributions, expressed through Wilson coefficients, from long-distance hadronic effects. Violations of LU could imply new physics (NP) beyond the SM, prompting searches for deviations in specific $b$-hadron decays into different lepton generations.

Experimental Approach and Observables

Lepton universality is tested by measuring ratios of branching fractions, specifically:

• $R_{D}$ and $R_{D^*}$: These ratios pertain to semitauonic decays where differential measurements may hint at NP contributions. Tensions in these ratios relative to SM expectations, compiled using theory (lattice QCD and heavy quark expansion) and past experiments, suggest potential deviations.
• $R_{K}$ and $R_{K^*}$: These ratios involve $b\to s\ell^+\ell^-$ transitions and their measured values have shown deviations below unity, contrary to SM predictions, especially in LHCb results. Such observations have sparked a significant debate on the role of unexplained dynamics or contributions from virtual particles like leptoquarks or $Z'$ bosons.

Deviations and Interpretation

The analysis further explores interpreting these discrepancies using effective field theory (EFT). Studies scrutinizing variations in Wilson coefficients provide insights into potential NP. Specific scenarios, such as left-handed or right-handed NP contributions and complex scalar or vector interactions, are discussed. Models like those considering leptoquark exchanges seek to address both tree and loop-level deviations simultaneously, offering a unified approach to hints of NP.

Prospective Analyses and Impact

The paper projects future advancements in improving statistical and systematic measurements with upcoming experiments at Belle-II and LHCb. With these experiments set to record orders of magnitude larger datasets, significant advancements in understanding or refuting LU deviations in $b$-hadron decays are anticipated. The expectation is that any confirmations of LU violations would not only necessitate extensive reinterpretations of the SM but also guide the construction of new theoretical frameworks.

Conclusion

Overall, the paper effectively summarizes the current landscape of LU tests in semileptonic $\mathbf{B}$ decays. By presenting the interplay between experimental results, theoretical predictions, and potential implications for NP, it establishes a crucial direction for future research in particle physics, with a focus on understanding the fundamental symmetries and interactions. This pursuit remains vital, as it may unveil new exotic particles or symmetries shaping the universe.