NA62 collaboration at CERN reports refined measurement of rare particle decay

Furthering their work first presented at a CERN EP seminar in 2024, the NA62 collaboration has announced new results at the 2026 La Thuile conference that dramatically reduce uncertainty in the measurement of an extremely rare particle decay.

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. 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 very little uncertainty. This 'theoretical cleanliness' makes this particular kaon decay extremely sensitive to new physics beyond the Standard Model, but with a predicted branching ratio of less than one in 10 billion, this process is extremely rare and very challenging to observe.

Professor Cristina Lazzeroni FRS, Professor in Particle Physics, University of Birmingham, said:

"The exquisite precision of this result really shows the potential of NA62 for precision measurements. We have more data available and even more data being collected this year, and we expect to refine the measurement further in the near future. Thanks to the extreme rarity and theoretical precision of this process, when we use all the NA62 data, we will be able to say more definitely if this process really follows the prediction of the Standard Model and to constrain more stringently a model of new physics."

The NA62 experiment was designed to study the K⁺→π⁺νν process in depth and therefore produces a lot of kaons, which is why it is also known as the kaon factory. The kaons are created by firing a high-intensity beam of protons from the Super Proton Synchrotron at a beryllium target. This produces nearly a billion particles every second, of which around 6% are kaons whose decay products can be studied in great detail using the NA62 detectors.

In 2024, the NA62 Collaboration reported observing this process with a statistical significance of five standard deviations, the gold standard in particle physics for claiming a discovery. Now, the researchers have included data recorded in 2023–24 and have used improved data analysis techniques based on cutting-edge machine learning algorithms. This, in combination with the previous data taken since the experiment began, has significantly refined their study of this ultra-rare kaon decay.

With the full dataset, the NA62 Collaboration obtained an updated value of the K⁺→π⁺νν branching ratio of 9.6^+1.9−1.8 × 10⁻¹¹, with an uncertainty 40% smaller than before.

Given the current precision, the kaon decay appears to occur as predicted by theory and provides powerful constraints on new physics beyond the Standard Model.