The Renaissance of Kaon Flavour Physics

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 $K^{+}\to {\pi }^{+}\nu \bar{\nu }$ and $K_L\to {\pi }^0\nu \bar{\nu }$ that are very sensitive to new physics (NP) and are theoretically very clean. Simultaneously the role of ${\epsilon }^{\prime }/\epsilon$, ${\epsilon }_K$, $\Delta M_K$, $K_L\to {\mu }^{+}{\mu }^{-}$ 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 ${\epsilon }^{\prime }/\epsilon$ 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 ${\epsilon }_K$ 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 ${\epsilon }^{\prime }/\epsilon$,and $\Delta F=2$ transitions and summarizes possible implications of simultaneous anomalies in ${\epsilon }^{\prime }/\epsilon$ and ${\epsilon }_K$ for $K^{+}\to {\pi }^{+}\nu \bar{\nu }$ and $K_L\to {\pi }^0\nu \bar{\nu }$ and $\Delta M_K$ in models with tree-level $Z$ and $Z^{\prime }$ exchanges. The anomalies in ${\epsilon }^{\prime }/\epsilon$ and ${\epsilon }_K$, 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.

Comments: 22 pages, 7 Figures, invited talk presented at Beauty 16, Marseille, France, May 2-6, 2016. To be published in Proceedings of Science. FLAVOUR(267104)-ERC-122, V2: One Table, references and clarifying comments added