Abstract
Historically important in the development of the Standard Model (SM) of particle physics, rare kaon decays are still a privileged tool for looking beyond it. The main reasons to continue the study of rare kaon decays are to test the CKM quark-mixing and CP-violation paradigm, to make quantitative comparisons with the B sector, and to search for explicit violations of the SM. Current research on rare kaon decays focuses mostly on [Formula: see text] decays, which are predicted with good accuracy within the SM and beyond. Experimentally, these decays, especially that of the charged kaon, have a long history. Their theoretical importance is matched only by their experimental difficulty. This article reviews the progress of the past 10 years, describes the state of the art, and looks toward future perspectives.
Highlights
Following the breakthrough of GIM, which explained that flavor-changing neutral currents (FCNCs) are strongly suppressed at loop level, it was realized that rare kaon decays such as KL0 → μ+μ−, K → π νν, K → γ γ, and others that exhibit induced | S| = 1 transitions are theoretically interesting because they allow the direct observation of higher-order electromagnetic and weak interactions [20]
The current picture is that each manifestation of CP violation is compatible with the idea that the complex CKM phase is the sole source of CP violation
It is clear that the search for new sources of CP violation is a compelling effort and that the study of as many systems as possible should be strongly pursued
Summary
We often lament that the SM passes every experimental test, but we tend to forget that the SM was not always the same; it has grown and incorporated, step-by-step, the discoveries made along the way. The discovery of neutral currents in neutrino scattering [15] and increasingly stringent limits on the absence of FCNCs led to the consolidation of the SM with rare kaon decays as a tool to test it in a significant way. Following the breakthrough of GIM, which explained that FCNCs are strongly suppressed at loop level, it was realized that rare kaon decays such as KL0 → μ+μ−, K → π νν , K → γ γ , and others that exhibit induced | S| = 1 transitions are theoretically interesting because they allow the direct observation of higher-order electromagnetic and weak interactions [20]. One might be tempted to conclude that with the advent of specialized B physics experiments such as the B factories and LHCb at the LHC collider, the role of kaons is exhausted This is not the case; we still need to overconstrain the CKM matrix with precise determinations of its parameters extracted independently using different quark systems. The K → π ννones described offer a interesting combination of theoretical cleanliness and experimental challenge
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