Scientists discover extremely rare particle decay

“We have found statistical evidence for one of the rarest processes in particle physics: a charged kaon decaying into a charged pion and a neutrino-anti neutrino pair,” says Marchevski. The data, collected by the NA62 experiment at CERN, marks the first time this rare process has been measured with a statistical significance of five standard deviations – meaning the collaboration can claim a discovery. While earlier experiments had hinted at the existence of this process, the new measurement provides definitive evidence and sets a new milestone in particle physics.

The process of a charged kaon decaying into a charged pion and a neutrino-anti-neutrino pair (also written K+ ➝ π+ν ν̅ ) is extremely rare. The Standard Model of particle physics predicts that only one in 10 billion kaons with a positive charge decays this way. At the same time, it is a very promising and informative channel – “the golden channel in flavour physics,” Marchevski says – because with its help, physicists can spot new physics very quickly if the data deviates from the precise Standard Model prediction. This means anything that is new, unexpected and extremely rare can be stress-tested against it very reliably. In contrast, if there isn’t anything new or unexpected it still leads to a much better understanding of the Standard Model and constrains large parameter space where physics beyond the Standard Model could hide.

Reaching this milestone wasn’t exactly a walk in the park. The search for this decay began nearly 50 to 60 years ago, with successive experiments gradually pushing the limits closer and closer towards the Standard Model's prediction, but never with enough data to claim a discovery. For this analysis, the NA62 team collected data between 2016 and 2023, a formidable task in itself. Operating at the cutting edge of detector technology, the NA62 setup demands very high efficiency and minimal downtime. Moreover, researchers must ensure precise event reconstruction and maintain the highest quality of the collected data. In the experiment's early stages, it was uncertain whether such ambitious targets could even be met.

In order to avoid confirmation bias in their analyses Marchevski, his small SNF-grant-funded team and the rest of the collaboration “needed to pay very close attention to our data and question everything that came in”, he explains. Because this particular kaon decay happens so rarely, and when it does, it’s only the pion that gives it away with the neutrino pair escaping undetected, the scientists had to suppress the background from the fixed-target collisions by twelve orders of magnitude.

“Our result is evidence that the technique works,” Marchevski says. With a value of 13.0-2.9+3.3×10-11 for the fraction of K+ that decay in this way, the result is 50% above what is predicted in the Standard Model, but within the framework of the overall uncertainty it is still a fit. “It just shows that we are still limited by statistics – we need more data to get the precision even higher and reduce the uncertainty.” NA62 is set to run until the end of 2025, with a possible extension into 2026, which should already triple the statistics, but the collaboration has plans for a detector upgrade for even better results.

The collaboration presented their result at a seminar held at CERN and hope to publish the according paper in the next months.

barbara Warmbein (for CHIPP)