New measurement of $K^{+}\to {\pi }^{+}\nu \bar{\nu }$ branching ratio at the NA62 experiment

Abstract

The ultra-rare decay $K^{+}\to {\pi }^{+}\nu \bar{\nu }$ is a golden mode in flavor physics. The Standard Model prediction for its branching ratio is below $10^{-10}$. This decay mode is highly sensitive to new physics models at mass scales up to $\mathcal{O}(100\mathrm{TeV})$. The NA62 experiment at CERN SPS is designed to measure this decay mode. A preliminary result of the branching ratio measurement using data collected in 2023--2024 is presented. With the new dataset, the NA62 experiment doubled its signal sample while reducing the background in proportion. Combining the data collected in 2016--2024, the branching ratio is measured to be $\mathcal{B}(K^{+}\to {\pi }^{+}\nu \bar{\nu })=\left(9.6_{-1.8}^{+1.9}\right)\times 10^{-11}$. The result is compatible with the Standard Model prediction with a precision better than $20\%$.

6 pages, 5 figures, contribution to the 2026 Electroweak session of the 60th Rencontres de Moriond