K → μ⁺μ⁻ as a third kaon golden mode

Abstract Recent progress has demonstrated that the K → μ + μ − decay carries clean short-distance information, attainable from a measurement of time-dependence sensitive to KL − KS interference effects. We review the ingredients that go into this proposed extraction, and discuss the sensitivity to the CKM parameter η ¯ as well as to various NP scenarios.