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

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 $\mathcal{O}(100\mathrm{T}\mathrm{e}\mathrm{V})$. 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 $20\mathrm{\%}$.