$K\to \mu^{+} \mu^{-}$ as a third kaon golden mode

Abstract

Recent progress has demonstrated that the $K\to\mu^+\mu^-$ decay carries clean short-distance information, attainable from a measurement of time-dependence sensitive to $K_L-K_S$ interference effects. We review the ingredients that go into this proposed extraction, and discuss the sensitivity to the CKM parameter $\bar\eta$ as well as to various NP scenarios.