Rare Kaon Decays

TL;DR

This paper reviews the current experimental efforts to measure rare kaon decays, highlighting their importance for testing the Standard Model and searching for new physics, including dark particles, with recent results from KOTO and NA62 experiments.

Contribution

It summarizes recent experimental results on rare kaon decays and discusses future prospects for these measurements in probing new physics and dark particles.

Findings

KOTO set a limit on $K_L^0 o \pi^0 u ar{ u}$ decay, close to Standard Model predictions.

NA62 measured $K^+ o \pi^+ u ar{ u}$ decay consistent with the Standard Model.

Future experiments aim to improve sensitivity to rare decay modes and search for dark particles.

Abstract

The experimental status of the $K \to \pi \nu \bar{\nu}$ search is presented. The $K \to \pi \nu \bar{\nu}$ decay is sensitive to New Physics because it is theoretically pristine and highly suppressed. The $K_L^0 \to \pi^0 \nu \bar{\nu}$ search is performed by the KOTO experiment and a branching fraction limit of $\mathcal{B}(K_L^0 \to \pi^0 \nu \bar{\nu})$ $<$ 3.0 $\times$ 10$^{-9}$ (90% confidence level) was set. This limit is $\mathcal{O}(100)$ times larger than the Standard Model prediction. The $K^+ \to \pi^+ \nu \bar{\nu}$ search is performed by the NA62 experiment and $\mathcal{B}(K^+ \to \pi^+ \nu \bar{\nu})$ $=$ (10.6 $^{+4.0}_{-3.5}$ $|_{\text{stat.}}$) $\pm$ 0.9$_{\text{syst.}}$) $\times$ 10$^{-11}$ (68% confidence level) was set. This shows an agreement with the Standard Model. Both measurements can also be used to search for the dark particle $X$ via the $K \to \pi X$ decay. A projection of the $K \to \pi \nu \bar{\nu}$ search in the future is also given.