Standard Model and New Physics contributions to $K_L$ and $K_S$ into four leptons

Abstract

We study the $K_L$ and $K_S$ decays into four leptons ($e\bar{e}e\bar{e}, \, \mu\bar{\mu}\mu\bar{\mu},\, e\bar{e}\mu\bar{\mu}$) where we use a form factor motivated by vector meson dominance, and show the dependence of the branching ratios and spectra from the slopes. A precise determination of short distance contribution to $K_L\to \mu\mu$ is affected by our ignorance on the sign of the amplitude $\mathcal{A}(K_L\to \gamma\gamma)$ but we show a possibility to measure the sign of this amplitude by studying $K_L$ and $K_S$ decays in four leptons. We also investigate the effect of New Physics contributions for these decays.Comment: Improvements of the text and references adde