First search for $K^+\to\pi^0\pi\mu e$ decays

TL;DR

This paper reports the first search for specific lepton number and lepton flavour violating decays of the charged kaon, setting new upper limits on their branching ratios based on data from the NA62 experiment at CERN.

Contribution

It presents the first experimental search for these rare kaon decays and establishes upper limits on their branching ratios, advancing the understanding of lepton number and flavour violation.

Findings

No evidence of the decays was observed.

Upper limits on branching ratios were set at 90% CL.

Results improve constraints on new physics models.

Abstract

The first search for the lepton number violating decay $K^+\to\pi^0\pi^-\mu^+e^+$ and lepton flavour violating decays $K^+\to\pi^0\pi^+\mu^-e^+$, $K^+\to\pi^0\pi^+\mu^+e^-$ has been performed using a dataset collected by the NA62 experiment at CERN in 2016-2018. Upper limits of $2.9\times 10^{-10}$, $3.1\times 10^{-10}$ and $5.0\times 10^{-10}$, respectively, are obtained at 90% CL for the branching ratios of the three decays on the assumption of uniform phase-space distributions.