Hints for new sources of flavour violation in meson mixing

TL;DR

Recent lattice results on meson mixing show discrepancies with Standard Model predictions, suggesting potential new sources of flavour violation and breaking of flavour universality, especially in constrained minimal flavour violation models.

Contribution

The paper analyzes tensions in meson mixing data within constrained minimal flavour violation models and explores implications for new sources of flavour violation and universality breaking.

Findings

Significant tension between measured and predicted mass differences in B meson mixing.

Breaking flavour universality can resolve the observed tensions.

Implications for $U(2)^3$ flavour models are discussed.

Abstract

The recent results by the Fermilab Lattice and MILC collaborations on the hadronic matrix elements entering $B_{d,s}-\bar B_{d,s}$ mixing show a significant tension of the measured values of the mass differences $\Delta M_{d,s}$ with their SM predictions. We review the implications of these results in the context of Constrained Minimal Flavour Violation models. In these models, the CKM elements $\gamma$ and $|V_{ub}|/|V_{cb}|$ can be determined from $B_{d,s}-\bar B_{d,s}$ mixing observables, yielding a prediction for $\gamma$ below its tree-level value. Determining subsequently $|V_{cb}|$ from the measured value of either $\Delta M_s$ or $\varepsilon_K$ gives inconsistent results, with the tension being smallest in the Standard Model limit. This tension can be resolved if the flavour universality of new contributions to $\Delta F = 2$ observables is broken. We briefly discuss the case of $U(2)^3$ flavour models as an illustrative example.