Lepton universality violation and lepton flavor conservation in $B$-meson decays (1505.05164v1)

Abstract: Anomalies in (semi)leptonic $B$-meson decays present interesting patterns that might be revealing the shape of the new physics to come. In order to understand the experimental data, we explore symmetry arguments that lead to the hypothesis of minimal flavor violation. In particular, under the assumption of negligible neutrino mass effects in charged lepton processes, the presence of lepton universality violation without lepton flavor violation naturally arises. This can account for a deficit of $B^+\to K^+\mu\mu$ over $B^+\to K^+ee$ decays with new physics coupled predominantly to muons and a new physics scale of a few TeV. A prediction of this scenario is the modification of processes involving the third generation. In particular, accounting for the above ratio implies a large enhancement, by a factor $\sim10^3$ with respect to the standard model, of all the $b\to s\tau\tau$ decay rates. Although these are still below current experimental limits, they should be easily at reach in future experiments at $B$-factories. Another important consequence is the prediction of sizable effects in charge-current $B$ tauonic decays which could also explain the enhancements that have been observed in the $B\to D^{(*)}\tau\bar \nu$ and $B^-\to\tau^-\bar \nu$ decays. For the most part, the study is carried out in an effective field theory framework with an underlying $SU(2)_L\times U(1)_Y$ symmetry that emphasizes the model-independent correlations arising between low- and high-energy observables. For example, a connection between $B$-decays and top physics is pointed out. To complement the discussion, all possible (spin 0 and 1) leptoquark models are matched to the low-energy field theory so that the effective analysis can be used to survey these candidates for new physics. These models also serve as concrete examples where the hypotheses of this work can be implemented.

• The paper demonstrates that the R_K anomaly shows a 2.6σ deviation from Standard Model predictions, indicating new muon-specific interactions.
• The paper employs an effective field theory framework to reveal that minimal flavor violation can explain lepton universality violation without triggering lepton flavor violation.
• The paper forecasts an approximate 10^3 enhancement in b→sττ decay rates, offering clear predictions for validation in future B-factory experiments.

Lepton Universality Violation and Lepton Flavor Conservation in $B$-Meson Decays

This paper presents an examination of anomalies within the (semi)leptonic decays of $B$-mesons, focusing on the potentials these anomalies may hold for unveiling novel physics phenomena. The paper scrutinizes the constraints imposed by symmetry principles that could result in minimal flavor violation (MFV) and discusses the implications of lepton universality violation (LUV) occurring without accompanying lepton flavor violation (LFV). Such findings are contextualized within the effective field theory (EFT) framework, elaborating on model-independent correlations bridging high- and low-energy observables.

Overview of Key Findings

The authors explore the observed disparities between the decay rates of $B^+\rightarrow K^+\mu^+\mu^-$ and $B^+\rightarrow K^+e^+e^-$. The anomaly, known as the $R_K$ measurement, indicates a 2.6\sigma deviation from the Standard Model (SM) expectation of unity, signaling lepton universality violation. This research anchors on elucidating these deviations through the prism of minimal flavor violation, postulating that LUV might arise without incurring LFV in charged lepton processes.

The theoretical framework underpinning this investigation solidifies within an effective field theory context, invoking the $SU(2)_L\times U(1)_Y$ symmetry. The analysis unveils that the $R_K$ anomaly can be rationalized by introducing new physics preferentially coupled to muons. Additionally, it anticipates a substantial enhancement in $b\rightarrow s\tau\tau$ decay rates, which, although beneath current experimental sensitivities, may become falsifiable with future endeavors at $B$-factories.

Noteworthy Implications

• Lepton Universality Violation: The characterized $R_K$ anomaly implies a nuanced breach of lepton universality against its Standard Model values. The declaration of LUV potentially confined to muonic processes ascribes to a suppression of lepton flavor violation by virtue of the underlying symmetry fundamentals.
• Enhanced Decay Rates: The theoretical predictions indicate an amplification of $b\to s\tau\tau$ decay channels by an approximate factor of $10^3$ relative to the Standard Model. Although currently elusive to experimental confirmation, these predictions hold significant promise for future $B$-factory experiments with enhanced detection capabilities.
• Interconnected Observables: The correlation established between neutral and charge-current semileptonic decays of $B$-mesons and top quark physics predicates a robust testing ground for the studied hypotheses, particularly through observable effects in $B\to D^{(*)}\tau\bar{\nu}$ and $B^-\to\tau^-\bar{\nu}$ decays.

Prospects and Future Research

The propositions presented entail both theoretical enrichment and experimental prospects requiring validation. Future experiments at $B$-factories and high-energy colliders will be pivotal in discerning these theoretical models, potentially unveiling new physics scales on the order of a few TeV. Moreover, exploring extensions of the present models through scalar and vector leptoquark contributions to the SMEFT framework could yield further granular insights into anomaly-induced new physics interactions.

This discourse provides a substantiated theoretical backdrop elucidating how directional new physics implementation—evinced through LUV without LFV in $B$-processes—can provoke a revisionary standpoint to leverage the established tensions with the Standard Model, ultimately fostering advancements in our comprehension of particle interactions.