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 $\mathcal{B}(K^+\to\pi^+\nu\bar\nu) = \left(9.6^{+1.9}_{-1.8}\right)\times10^{-11}$. The result is compatible with the Standard Model prediction with a precision better than $20\%$.