Geneva, 25 September 2024. At a seminar held at CERN this week, the NA62 collaboration reported the unequivocal confirmation of the ultra-rare decay of a positively charged kaon into a positively charged pion and a neutrino–antineutrino pair. Experiments including NA62 have previously measured and seen evidence of this process, but this is the first time it has been measured with a statistical significance of five standard deviations, crossing the threshold traditionally required to claim a discovery in particle physics.

Denoted by K⁺→π⁺νν, this decay is among the rarest particle processes ever observed: in the Standard Model of particle physics, less than one in 10 billion positively charged kaons are predicted to decay in this way.

“This observation is the culmination of a project that started more than a decade ago,” says NA62 spokesperson Giuseppe Ruggiero. “Looking for effects in nature that have probabilities of happening of the order of 10^-11 is both fascinating and challenging. After rigorous and painstaking work, we have finally seen the process NA62 was designed and built to observe.”

But why are physicists looking for a process that occurs so rarely? The reason is that theoretical models suggest that the K⁺→π⁺νν decay is extremely sensitive to deviations from the Standard Model prediction, making it one of the most interesting processes to search for evidence of new physics beyond the Standard Model.

Analysing data collected by the NA62 detector between 2016 and 2022, the NA62 researchers measured the fraction of K⁺ that decay in this way to be 13.0 _(+3.3)
^(-2.9) × 10^(-11). With a relative precision of 25%, this is the most precise measurement of the K⁺→π⁺νν decay to date.

The result is about 50% larger than the Standard Model prediction but is compatible with it given the overall uncertainty. With data taking ongoing, NA62 is set to be able to test the possibility of new physics in this decay within the next few years.

“Searching for hints of new physics in this decay requires more data, but this result is a leap forward and further strengthens the strong interest in this line of research,” says NA62 physics coordinator Karim Massri.

In the NA62 experiment, kaons are produced by slamming a high-intensity proton beam from CERN’s Super Proton Synchrotron into a stationary target. As a result, almost a billion secondary particles are produced each second, and these fly into the NA62 detector. Of these particles, about 6% are positively charged kaons. NA62 precisely detects the decay products of kaons, identifying and measuring all the particles produced except the neutrinos, whose presence is deduced from their missing energy.

Crucial to this result was the data from 2021 and 2022, which was taken following the completion of detector upgrades that enabled NA62 to operate at 30% higher beam intensities. Combined with improvements to data-analysis techniques, these hardware upgrades enabled the collection of signal candidates 50% faster than before, while adding new tools to suppress the background processes that could mimic the K⁺→π⁺νν decay.

“This measurement relies on identifying the one-in-10-billion K⁺ decay that is our signal and making sure it is not one of the other 9 999 999 999 decays that can mimic the signal,” says lead data analyst Joel Swallow. “The whole NA62 collaboration has made this almost impossible result possible.”