Searches for lepton number violating $K^+\to\pi^-(\pi^0)e^+e^+$ decays

TL;DR

This paper reports on searches for lepton number violating decays of charged kaons, setting new upper limits on their branching fractions and providing the first limit for one decay mode, using data from the NA62 experiment.

Contribution

The study presents the first limit on the $K^+ o ext{pi}^- ext{pi}^0 e^+ e^+$ decay and improves existing limits on $K^+ o ext{pi}^- e^+ e^+$ decays, advancing constraints on lepton number violation.

Findings

Upper limit of $5.3\times 10^{-11}$ on $K^+\to\pi^- e^+ e^+$ decay

Upper limit of $8.5\times 10^{-10}$ on $K^+\to\pi^-\pi^0 e^+ e^+$ decay

First limit established for $K^+\to\pi^-\pi^0 e^+ e^+$ decay

Abstract

Searches for lepton number violating $K^+\to\pi^-e^+e^+$ and $K^+\to\pi^-\pi^0e^+e^+$ decays have been performed using the complete dataset collected by the NA62 experiment at CERN in 2016-2018. Upper limits of $5.3\times 10^{-11}$ and $8.5\times 10^{-10}$ are obtained on the decay branching fractions at 90% confidence level. The former result improves by a factor of four over the previous best limit, while the latter result represents the first limit on the $K^+\to\pi^-\pi^0e^+e^+$ decay rate.