$K\to {\mu }^{+}{\mu }^{-}$ beyond the standard model

Abstract

We analyze the New Physics sensitivity of a recently proposed method to measure the CP-violating $\mathcal{B}(K_S\to {\mu }^{+}{\mu }^{-}{)}_{\ell =0}$ decay rate using $K_S-K_L$ interference. We present our findings both in a model-independent EFT approach as well as within several simple NP scenarios. We discuss the relation with associated observables, most notably $\mathcal{B}(K_L\to {\pi }^0\nu \bar{\nu })$. We find that simple NP models can significantly enhance $\mathcal{B}(K_S\to {\mu }^{+}{\mu }^{-}{)}_{\ell =0}$, making this mode a very promising probe of physics beyond the standard model in the kaon sector.