New Measurement of the \(K^+→ π ^+ν ν \) Branching Ratio at the NA62 Experiment

A new measurement of the \(K^+→ π ^+ν ν \) decay by the NA62 experiment at the CERN SPS is presented, using data collected in 2021 and 2022. Combining the NA62 datasets from 2016–2022, the decay branching ratio is measured to be: \(BR = (13.0+3.3-3.0) × 10⁻¹¹\). For the first time, the decay is observed with a significance exceeding \(5σ \). The reinterpretation of the result as a \(K^+→ π ^+ X\) search, where \(X\) is a feebly interacting particle, is also discussed. Abstract Published by the Jagiellonian University 2026 authors