Relatore
Descrizione
The $K^+\rightarrow\pi^+\nu\bar\nu $ decay is a golden mode for flavour physics. Its branching ratio
is predicted with high precision by the Standard Model to be less than $10^{−10}$,
and this decay mode is highly sensitive to indirect effects of new physics up to
the highest mass scales. A new measurement of the $K^+\rightarrow\pi^+\nu\bar\nu $ decay by the
NA62 experiment at the CERN SPS is presented, using data collected in 2021
and 2022. This new dataset was collected after modifications to the beamline
and detectors and at a higher instantaneous beam intensity with respect to the
previous 2016–2018 data taking. Using the NA62 datasets from 2016–2022, a
new measurement of $BR(K^+\rightarrow\pi^+\nu\bar\nu) = 13.0^{+3.3}_{-2.9}\times 10^{-11} $ is
the first time the $K^+\rightarrow\pi^+\nu\bar\nu$ decay is observed with a significance exceeding $5\sigma$.
