On Sunday 11th of March, NA62 collaboration presented in Moriond the first results of the K+→π+νν analysis. This decay channel has a branching ratio in the SM of (8.4±1.0)×10-11 with solid theoretical predictions dominated by short distance contributions and with the hadronic uncertainties well under control. This channel is a golden one in the search of New Physics as any deviation from the Standard Model prediction will be a signature of BSM physics. A seminar, with all details of the analysis was held at CERN last Tuesday 27th of March.
The analysis presented in is based in the study of a sample of 1.21×1011 K+ decays recorded during the 2016 run. Two independent blind analysis have been done in parallel and both have found one event, the same.
With the K+. decays analyzed the expectations were of 0.267±0.032 signal events and 0.15±0.19 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+→π+νν)< 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 decay in-flight technique takes profit of the Lorentz boost of 75 GeV beam coming from SPS, allowing to concentrate all detectors in the forward region. This technique is quite different to the one used in previous dedicated experiments in Brookhaven in which the K+ were stopped in a target and their decays studied at rest. The decay in-flight technique has the advantage with respect to the decay at rest technique to allow a better control and hence reduce the background.
Essential in this first result is the excellent performance of the experimental setup, based on a precise kinematic reconstruction of both kaons and their decays, a 100ps time resolution and an excellent particle identification. As an example, in the figure below it is shown the pion identification of the K+→π+νν candidate with the pion, muon and electron predictions:
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 the whole statistics has been analyzed.