[Paper Review] Flavour Anomalies

Antonio Pich|arXiv (Cornell University)|Nov 14, 2019

Particle physics theoretical and experimental studies19 references4 citations

TL;DR

This paper reviews observed deviations in $b \to s\ell^+\ell^-$ and $b \to c\ell\bar{\ell}}$ transitions from Standard Model predictions, analyzing them via a model-independent effective Lagrangian approach. It finds that the direct CP-violating parameter $\varepsilon_K'/\varepsilon_K$ is consistent with experiment when all theoretical inputs are properly included, while other flavour anomalies—such as $D^0$ CP asymmetries—remain under scrutiny.

ABSTRACT

The experimental data on $b o c au\bar u_ au$ and $b o s\ell^+\ell^-$ transitions exhibit sizeable discrepancies with the Standard Model expectations. We present an overview of the present status and discuss possible interpretations within a model-independent effective Lagrangian approach. We also briefly elaborate on some other claimed flavour anomalies such as the recently observed CP asymmetry in $D^0$ decays or the $K^0 o \pi\pi$ ratio $\varepsilon_K'/\varepsilon^{\phantom{'}}_K$. The Standard Model prediction for the direct $CP$-violating ratio $\varepsilon_K'/\varepsilon^{\phantom{'}}_K$ agrees with its measured value, once all theoretical ingredients are correctly taken into account.

Motivation & Objective

• To assess the current status of experimental anomalies in $b \to s\ell^+\ell^-$ and $b \to c\ell\bar{\ell}}$ transitions.
• To interpret these anomalies using a model-independent effective field theory framework.
• To evaluate the significance of other claimed flavour anomalies, including $D^0$ CP asymmetries and the $\varepsilon_K'/\varepsilon_K$ ratio.
• To determine whether the observed $\varepsilon_K'/\varepsilon_K$ discrepancy is resolved by improved theoretical calculations.

Proposed method

• Employing an effective Lagrangian approach to describe new physics contributions to $b \to s\ell^+\ell^-$ and $b \to c\ell\bar{\ell}}$ transitions.
• Using experimental data on $B$ and $D^0$ decays to constrain effective operators in the Lagrangian.
• Applying theoretical inputs such as QCD corrections and hadronic matrix elements to compute $\varepsilon_K'/\varepsilon_K$.
• Comparing theoretical predictions for $\varepsilon_K'/\varepsilon_K$ with the measured value to assess consistency.
• Evaluating the impact of new physics contributions on $CP$ asymmetries in $D^0$ decays.
• Assessing the robustness of the $\varepsilon_K'/\varepsilon_K$ result under different theoretical assumptions.

Experimental results

Research questions

• RQ1 Are the observed deviations in $b \to s\ell^+\ell^-$ and $b \to c\ell\bar{\ell}}$ decays consistent with new physics beyond the Standard Model?
• RQ2 How do model-independent effective field theory methods constrain possible new physics contributions to these transitions?
• RQ3 What is the role of theoretical uncertainties in the prediction of $\varepsilon_K'/\varepsilon_K$, and do they resolve the apparent discrepancy?
• RQ4 Is the recently observed $CP$ asymmetry in $D^0$ decays indicative of new physics or explainable within the Standard Model?
• RQ5 Does the measured value of $\varepsilon_K'/\varepsilon_K$ agree with the Standard Model when all theoretical inputs are correctly accounted for?

Key findings

• The direct $CP$-violating ratio $\varepsilon_K'/\varepsilon_K$ is found to be consistent with the measured value when all theoretical inputs are properly included.
• No significant discrepancy remains between the Standard Model prediction and the measured $\varepsilon_K'/\varepsilon_K$ once theoretical uncertainties are fully accounted for.
• The observed anomalies in $b \to s\ell^+\ell^-$ and $b \to c\ell\bar{\ell}}$ transitions persist as potential signals of new physics.
• The $D^0$ decay $CP$ asymmetry remains a possible indicator of new physics, though not yet conclusively established.
• The effective Lagrangian approach successfully captures the phenomenology of flavour-changing neutral currents without assuming a specific UV completion.
• The analysis shows that the current data do not yet rule out the Standard Model for $\varepsilon_K'/\varepsilon_K$, despite earlier tensions.