New result on BR(K^+ \to \pi^+ \nu \bar{\nu}) from NA62 experiment


A way to investigate the presence of new physics at very high energy, not accessible to LHC, is searching for deviations from the SM of quantities predicted very precisely within the SM, even at small energies.

One of these quantities is how often a charged Kaon decays to a charged pion and a neutrino-antineutrino pair (BR(Kpnn) in the following). This decay happens very rarely, less than once every 10^10 K^+ decays. For this reason a large Kaon dataset is needed. UCLouvain physicists and students participate in the NA62 experiment at the CERN, which is designed to measure BR(Kpnn) with a decay-in-flight technique and took data so far from 2016 to 2018. The result obtained with only the 2016 dataset (a small fraction of the full dataset) and presented last year proved the operation and reliability of the experiment, but was still far from the SM domain.

A new result has recently been presented at the KAON 2019 conference and in a CERN seminar (

The analysis based on the 2017 dataset, 10 times larger than the 2016 one, resulted in two event candidates in the signal regions. Considering 2016 and 2017 data, NA62 Collaboration has set an upper limit for the BR(Kpnn) of 1.85 x 10^-10 at 90% C.L., excluding, with this confidence level, the BSM hypotheses which foresee the BR(Kpnn)~2.2 times larger than the SM one. The ongoing analysis of 2018 dataset will improve the sensitivity while, in order to perform a measurement of this BR with the same precision reached by the theoretical computations, a new data taking period is needed and will start in 2021.


Published on September 24, 2019