Abstract: The K+ -> pi nu nu_bar 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+ -> pi+ nu nu_bar decay by the NA62 experiment at the CERN SPS is presented, using data collected in 2021 and 2022.
This new data-set 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 data-set from 2016-2022, a new measurement of the branching fraction ( 13.0 +3.3/-2.9 x 10^-11) is reported, and for the first time the K+ -> pi+ nu nu_bar decay is observed with a significance exceeding 5 sigma.
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