The presented result corresponds to the analysis of the 2018 data set. 17 events were found in the signal region. The combination of this result with the analysis of the 2016 and 2017 data sets (1 and 2 events observed) have allowed the NA62 collaboration to measure the branching ratio of this rare decay:
This new result, measured with 30% precision, is the most precise measurement of this process to-date. Although it is consistent with the Standard Model expectation, it still leaves room for the existence of physics beyond the Standard Model.
The NA62 collaboration is preparing a new data taking period in 2021-2024 with an improved beam line and detector allowing the measurement at higher intensities. This new run will allow NA62 to confirm the result obtained in 2018 and measure the branching ratio to 10% precision, comparable with the uncertainty on the theoretical prediction of this process.
UCLouvain has been a member of the NA62 collaboration since 2010 and has been heavily involved in the construction of the original detector as well as the upgrade of the detector for the 2021 run.