Kaon Physics in Europe

A review of selected recent results from experiments studying rare kaon decays in Europe is given. The results from the NA62 experiment at CERN are summarized in the first part of this report. The experiment produced a new preliminary upper limit on the \(K^{ + } \to \pi ^{ + }\nu \bar{\nu }\) branching ratio of 1.85 × 10^−10 at 90% CL, based on the data recorded in 2016 and 2017. NA62 also performed searches for production of heavy neutral leptons (N) in \(K^{ + } \to \ell ^{ + }N\) (\(\ell = e,\mu \)), and for lepton number violating processes \(K^{ + } \to \pi ^{ - }\ell ^{ + }\ell ^{ + }\) (\(\ell = e,\mu \)). No significant signal was observed in these searches, and the world-leading upper limits have been set. In the second part of this review, we summarize the first observation and the branching ratio measurement of the \(K^{ \pm } \to \pi ^{ \pm }\pi ^{0}e^{ + }e^{ - }\) decay by the NA48/2 experiment, a new preliminary measurement of the \(K_{S} \to \pi ^{ \pm }\mu ^{ \mp }\nu \) branching ratio by the KLOE experiment, and a new upper limit on the \(K_{S} \to \mu ^{ + }\mu ^{ - }\) branching ratio by the LHCb experiment.