The Revival of Kaon Flavour Physics

TL;DR

The paper discusses the renewed focus on kaon flavour physics, emphasizing new experimental measurements and theoretical advances that enhance the search for physics beyond the Standard Model, especially through rare decay processes and hadronic calculations.

Contribution

It highlights recent experimental results and theoretical improvements in kaon physics, proposing that these developments could reveal new physics beyond current models.

Findings

New measurements of rare kaon decay branching ratios by NA62 and KOTO.

Improved lattice QCD and large N calculations of hadronic matrix elements.

Hints of new physics contributions to $ ext{ε'} / ext{ε}$ from recent theoretical estimates.

Abstract

After years of silence we should witness in the rest of this decade and in the next decade the revival of kaon flavour physics. This is not only because of the crucial measurements of the branching ratios for the rare decays $K^+\to\pi^+\nu\bar\nu$ and $K_{L}\to \pi^0\nu\bar\nu$ by NA62 and KOTO that being theoretically clean and very sensitive to new physics (NP) could hint for new phenomena even beyond the reach of the LHC without any significant theoretical uncertainties. Indeed simultaneously the advances in the calculations of perturbative and in particular non-perturbative QCD effects in $\varepsilon'/\varepsilon$, $\varepsilon_K$, $\Delta M_K$, $K_L\to\mu^+\mu^-$ and $K_L\to\pi^0\ell^+\ell^-$ will increase the role of these observables in searching for NP. In fact the hints for NP contributing to $\varepsilon'/\varepsilon$ have been already 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. This talk summarizes in addition to this new flavour anomaly the present highlights of this field including some results from concrete NP scenarios.