Particles: this ultra-rare decay could upset physics

A very rare phenomenon has just been observed at CERN. The decay of a charged kaon into a pion and neutrinos is intriguing scientists.

This exceedingly rare process could open a new door to unknown physical phenomena. The NA62 experiment is at the heart of this discovery.


The kaon, a subatomic particle, decays into a pion, accompanied by a neutrino-antineutrino pair. This phenomenon is so rare that the Standard Model predicts less than one event per 10 billion kaons. This low probability fascinates physicists as it could reveal new physics beyond our current understanding. The NA62 team has designed specific equipment to observe this decay.

Cristina Lazzeroni, a professor at the University of Birmingham, emphasizes the importance of this observation. For her, it is one of the rarest ever recorded, with a level of certainty reaching the famed "5 sigma." Collaboration between researchers was essential to obtaining these results.

Kaons are produced using an intense proton beam accelerated by CERN's Super Proton Synchrotron and are isolated into a secondary particle beam directed toward the NA62 detector. This detector measures the decay products of the kaons with great precision, especially the pions, the only detectable decay products.

Giuseppe Ruggiero, from the University of Florence, reminds us that this discovery is the result of more than a decade of effort. According to him, the extremely low probability of this decay made the task difficult, but the result is a reward commensurate with the effort invested.

The data comes from experiments conducted between 2021 and 2022, following the introduction of technical improvements. Thanks to increased beam intensity and more refined analysis, the team was able to identify more potential signals while reducing measurement errors.

The NA62 team is focusing on this decay process because it is particularly sensitive to new physical theories. Although current results match the predictions of the Standard Model, a slight observed increase could suggest the existence of new particles.

The coming years will be decisive in confirming or disproving the existence of these new theories.

What is the Standard Model in particle physics?

The Standard Model is the theory that describes elementary particles and their fundamental interactions. It encompasses three of the four fundamental forces of the Universe: electromagnetism, the strong nuclear force, and the weak nuclear force. Gravity, however, is not included.

This theory organizes particles into two main categories: quarks and leptons. Quarks form protons and neutrons, while leptons include particles such as electrons and neutrinos. Bosons, such as the photon or Higgs boson, are responsible for the interactions between particles.

Although highly successful, the Standard Model does not explain everything. It does not account for gravity, dark matter, or dark energy, which drives scientists to search for new theories to complete or surpass this model.

What is the decay of a kaon?

The decay of a kaon is a process in which a kaon, an unstable subatomic particle, transforms into lighter particles. These transformations follow the laws of particle physics, particularly those described by the Standard Model. The kaon can decay into a pion, a neutrino, and an antineutrino.

This process is important because it occurs extremely rarely. Indeed, fewer than one kaon in 10 billion decays in this manner. Researchers are particularly interested because deviations from Standard Model predictions could reveal new particles or forces that are still unknown.