Séminaires LLR

# Measurement of the very rare $K^+ \to \pi^+ \nu \bar{\nu}$ decay from NA62

## by Francesco Brizioli (University of Perugia and INFN)

Europe/Paris
Description

The ultra-rare decay $K^+ \to \pi^+ \nu \bar{\nu}$, with a very precisely predicted branching ratio of less than $10^{-10}$, is one of the best candidates to reveal indirect effects of new physics at the highest mass scales. The NA62 experiment at CERN SPS is designed to measure $Br(K^+ \to \pi^+ \nu \bar{\nu})$ with a decay-in-flight technique, novel for this channel. NA62 took data in 2016, 2017 and 2018. Statistics collected in 2016 and 2017 allows NA62 to reach the Standard Model sensitivity for the $K^+ \to \pi^+ \nu \bar{\nu}$ search. The analysis of the 2018 data led to the most precise measurement ever performed, with more than $3 \sigma$ significance in terms of process observation. A statistical reinterpretation of the analysis gives also the possibility to set limits on $Br(K^+ \to \pi^+ X)$, where $X$ is a scalar or pseudo-scalar particle. The context of the measurement will be presented, the details of the analysis will be described, the results and the prospects will be illustrated.