On Sunday 11th of March, NA62 collaboration presented in Moriond the first results of the $K^+\to \pi^+ \nu\bar{\nu}$ analysis. This channel has a branching ratio in the SM of (8.4 ±1.0) × 10-11.
The analysis presented in Moriond is based in the study of a sample of 1.21× 10-11 K+ decays recorded during the 2016 run. Two independent blind analysis have been done in parallel and both have founded one event, the same.
With the K+ decays analyzed the expectations were of 0.267± 0.032 signal events and 0.15 ± 0.09 background events.
In the figure, you can see the two signal regions (boxes in red) with the event.
This result, allows to put a preliminary upper limit in the branching ratio:
BR($K^+\to \pi^+ \nu\bar{\nu}$) < 14 ×10-10 @ 95% CL
More important than the result, this analysis proves that the in-flight technique in which the collaboration has been working since many years works and it can have the background under control.
The analysis of 2017 data sample (x20 more statistics) has already started and in 2018, a long run (218 days) will start in April. It is expected to collect an even larger sample as in 2017. The collaboration aims to identify 20 events of this very rare channel once analyzed the whole statistics.