The Renaissance of Kaon Flavour Physics

TL;DR

This paper discusses the upcoming importance of kaon flavour physics in detecting new physics through rare decay measurements and improved SM estimates, highlighting recent anomalies and their implications.

Contribution

It provides a comprehensive overview of recent anomalies in kaon and B physics, exploring their potential as signals of new physics and discussing implications for models with Z and Z' exchanges.

Findings

Hints of anomalies in '/ and _K suggest new sources of CP violation.

Recent lattice QCD and large N dual QCD estimates support potential new physics signals.

Tensions in _K and  mass differences indicate possible deviations from the Standard Model.

Abstract

There is no doubt that in the coming years we will witness the renaissance of kaon flavour physics with crucial measurements of the branching ratios for the rare decays $K^+\rightarrow\pi^+\nu\bar\nu$ and $K_{L}\rightarrow\pi^0\nu\bar\nu$ that are very sensitive to new physics (NP) and are theoretically very clean. Simultaneously the role of $\varepsilon'/\varepsilon$, $\varepsilon_K$, $\Delta M_K$, $K_L\to\mu^+\mu^-$ and $K_L\to\pi^0\ell^+\ell^-$ in searching for NP will significantly increase through their improved estimates within the SM. In fact the hints for NP contributing to $\varepsilon'/\varepsilon$ have been signalled last year through improved estimates of hadronic matrix elements of QCD and electroweak penguin operators $Q_6$ and $Q_8$ by lattice QCD and large $N$ dual QCD approach. Also recent increased tensions between $\varepsilon_K$ and $\Delta M_{s,d}$ mass differences in $B_{s,d}^0-\bar B_{s,d}^0$ mixing within the SM and models with constrained MFV tell us that we should have tremendous fun in chasing NP through all these observables. The present talk discusses the hinted anomalies in $\varepsilon'/\varepsilon$,and $\Delta F=2$ transitions and summarizes possible implications of simultaneous anomalies in $\varepsilon'/\varepsilon$ and $\varepsilon_K$ for $K^+\rightarrow\pi^+\nu\bar\nu$ and $K_{L}\rightarrow\pi^0\nu\bar\nu$ and $\Delta M_K$ in models with tree-level $Z$ and $Z^\prime$ exchanges. The anomalies in $\varepsilon'/\varepsilon$ and $\varepsilon_K$, if confirmed, would signal new sources of CP violation. This is in contrast to the recent anomalies in $B$ physics that signal lepton flavour non-universalities and NP in CP-conserving observables. Highlights from recent flavour analyses of 331 models are briefly described.