The NA62 collaboration reported the unambiguous confirmation of the ultra-rare decay of a positively charged kaon into a positively charged pion and a neutrino-antineutrino pair. However, this is the first time it has been measured with a statistical significance of five standard deviations, which is traditionally required for a discovery in Particle Physics.
The decay, designated K+ ➝ π+νν̅, is the rarest particle process ever observed: According to the Standard Model of Particle Physics, less than one in 10 billion positively charged kaons will decay in this way.
“This observation is the culmination of a project that began more than a decade ago,” says NA62 spokesperson Giuseppe Ruggiero. “The search for effects in nature with a probability of occurrence in the order of 10-11 is both fascinating and challenging. After hard and careful work, we have finally seen the process for whose observation NA62 was designed and built.”
But why are the physicists looking for a process that is so rare? The reason: Theoretical models suggest that the K+→π+νν decay is extremely sensitive to deviations from the predictions of the Standard Model, making it one of the most interesting processes for searching for clues to new physics beyond the Standard Model.
“Finding clues to new physics requires even more data, but the current result is a major step forward and raises anticipation for future measurements,” says Rainer Wanke, head of the NA62 group at the Johannes Gutenberg-Universität Mainz.
When analyzing the data collected by the NA62 detector between 2016 and 2022, the NA62 researchers measured the proportion of K+ decaying in this way to be 13.0-2.9+3.3×10-11. With a relative accuracy of 25%, this is the most accurate measurement of the K+→π+νν decay to date. The group in Mainz contributed a 30-ton detector component to NA62 to separate π+- from the numerous µ+ particles, and is also responsible for the experiment’s computer farm.
The result is about 50% above the Standard Model prediction, but is consistent with it given the overall uncertainty. With ongoing data collection, NA62 will be able to test the possibility of new physics in this decay in the coming years.
In the NA62 experiment, kaons are generated by firing a high-intensity proton beam from CERN’s Super Proton Synchrotron onto a stationary target. This produces almost one billion secondary particles per second, which fly into the NA62 detector. Of these, about 6% are positively charged kaons. NA62 precisely detects the decay products of the kaons and identifies and measures all generated particles with the exception of the neutrinos, whose presence can be deduced from their missing energy.
Crucial to this result were the data from 2021 and 2022, which were recorded after the completion of detector upgrades that enabled NA62 to operate with 30% higher beam intensities. In combination with improved data analysis techniques, these hardware upgrades enabled 50% faster detection of signal candidates than before, while also adding new instruments to suppress background processes that could mimic the K+→π+νν decay.
“The measurement relies on identifying the one decay in 10 billion K+ decays that represents our signal and ensuring that it is not one of the other 9,999,999,999 decays that can mimic the signal,” says lead data analyst Joel Swallow. “The entire NA62 collaboration has made this almost impossible result possible.”
