1 in 10 billion: CERN’s breakthrough kaon decay discovery sparks new physics

Researchers at CERN have observed an exceptionally rare particle decay event, potentially paving the way to uncover new physics beyond the current understanding of fundamental particles and their interactions.

This decay is extraordinarily uncommon—according to the Standard Model (SM) of particle physics, which describes particle interactions, fewer than one in every 10 billion kaons undergo this specific decay.

The NA62 experiment was developed and optimized precisely to detect and study this elusive kaon decay process.

“This particular decay is extremely well predicted in the Standard Model. And it is sensitive to a large variety of theoretical models predicting the existence of new physics beyond the SM. Therefore, if we measure a deviation from the SM, it’s a clear sign of new physics,” Cristina Lazzeroni, Professor in Particle Physics at the University of Birmingham, told Interesting Engineering (IE) .

Studying kaon decay

Kaons are generated when a high-intensity proton beam from CERN’s Super Proton Synchrotron (SPS) collides with a fixed target, producing a stream of secondary particles.

Nearly a billion particles per second are directed into the NA62 detector, with around 6% being charged kaons. The detector meticulously identifies each kaon and measures its decay products, excluding neutrinos, which appear as missing energy in the data.

Lazzeroni tells IE, “ Kaons and pions are particles containing quarks. The fact that quarks are of different types (up, down, strange, charm, beauty, top) is called flavor.”

“The rarity of this decay has to do with the fact that, in it, there is a change in quark flavour that is mediated by Z boson and produces a pion and neutrinos. This can only happen with a rather elaborate process, hence its rarity.”

“Experimentally, NA62 detects the charged kaon, the charged pion but doesn’t detect the 2 neutrinos. So it’s “one particle in, one particle out and nothing else” sort of signature,” she further explains.

“The idea is to measure the K+ and the pi+ and to ensure that nothing else is missed or going lost or undetected. Any other K+ decay modes with lost or undetected particles could be a sort of spurious events, what we called ‘background’. So NA62 is making a big effort to veto in the detector everything (except neutrinos) and reject anything that doesn’t look like the signal.”

Deviations from the Standard Model

The rate at which kaons decay into a pion and two neutrinos has been measured at approximately 13 occurrences in every 100 billion kaons. While this aligns with the predictions of the Standard Model (SM), it is about 50% higher than expected.

This discrepancy could suggest the influence of unknown particles that enhance the probability of this decay. However, additional data is needed to validate this hypothesis.

Lazzeroni says about this result, “At present, the NA62 measurement is still compatible with the SM (at about 2sigma).”

“However, the 50% higher experimental value could be a hint of the presence of new physics. It is too early to say which particular model this could point to, if the discrepancy remains in the future. In general, it could point to the existence of new mediators (some possible ones are Z’ or leptoquarks).”

The NA62 experiment is actively gathering more data, and researchers hope to either confirm or exclude the possibility of new physics influencing this decay process in the coming years.

The findings were presented at a CERN EP seminar .