The NA62 Collaboration has dramatically reduced the uncertainty in its measurement of an extremely rare particle decay, in results just presented at the 2026 La Thuile conference.

The study of rare decays gives physicists the chance to probe the Standard Model of particle physics. Researchers can determine what is known as the branching ratio of a decay, which describes how many particles decay through a particular process as a fraction of the total number of decays that occur.

The branching ratio of the decay that the NA62 Collaboration has studied—the decay of a positively charged kaon into a positively charged pion and neutrino–antineutrino pair (written K⁺→π⁺νν)—can be predicted theoretically with a very high degree of precision. Thanks to this “theoretical cleanliness,” this particular kaon decay is extremely sensitive to new physics beyond the Standard Model but, with a predicted branching ratio of less than one in 10 billion, it is extremely rare and very challenging to observe.