Geneva, 25 September 2024 – In a landmark discovery at CERN, the NA62 collaboration has confirmed the observation of one of the rarest particle decays ever recorded: the decay of a positively charged kaon (K+) into a positively charged pion (π+), accompanied by a neutrino and an antineutrino (K+→π+νν). This breakthrough, achieved with a statistical significance of five standard deviations, represents the first unequivocal confirmation of this ultra-rare process, with odds of occurrence less than one in 10 billion according to the Standard Model of particle physics.
The significance of this discovery cannot be overstated. Although previous experiments had detected signs of this elusive decay, the NA62 experiment’s latest result is the first to meet the stringent statistical criteria required to claim a full discovery in particle physics.
Understanding the Ultra-Rare Kaon Decay (K+→π+νν)
The decay process observed by the NA62 collaboration is one of the rarest phenomena predicted by the Standard Model of particle physics. With an occurrence rate of fewer than one in 10 billion decays, K+→π+νν is a highly sought-after event for physicists. But why pursue a process that happens so infrequently?
Physicists are deeply interested in the K+→π+νν decay because it is exceptionally sensitive to deviations from the predictions of the Standard Model. This makes it a powerful tool for probing potential new physics and understanding phenomena beyond the established framework. Deviations from the predicted decay rate could hint at unknown forces or particles not yet accounted for, making it a key focus in the search for physics beyond the Standard Model.
Giuseppe Ruggiero, the spokesperson for the NA62 experiment, highlights the significance of the observation:
“This observation is the culmination of a project that started more than a decade ago. Looking for effects in nature that have probabilities of happening on 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.”
The NA62 Experiment: Pushing the Boundaries of Particle Physics
The NA62 experiment, located at CERN, is designed specifically to study rare kaon decays. Kaons are produced by firing a high-intensity proton beam from CERN’s Super Proton Synchrotron (SPS) into a stationary target, which generates nearly a billion secondary particles every second. Among these, approximately 6% are positively charged kaons, which are then studied in the NA62 detector.
The challenge is immense: only one in 10 billion kaons undergoes the K+→π+νν decay. The NA62 detector meticulously measures the decay products of kaons, with the neutrinos’ presence inferred from missing energy. To make this observation possible, the NA62 collaboration relies on cutting-edge hardware, which was recently upgraded to enhance beam intensity by 30%, and advanced data analysis techniques.
Joel Swallow, lead data analyst for NA62, underscores the complexity:
“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. The whole NA62 collaboration has made this almost impossible result possible.”
Results: A Leap Forward for Particle Physics
The data from the NA62 experiment between 2016 and 2022 allowed researchers to measure the fraction of K+ decays to be 13.0 +3.3/-2.9 × 10⁻¹¹, the most precise measurement of the K+→π+νν decay to date. Although this result is approximately 50% larger than the Standard Model prediction, it remains compatible given the current uncertainty.
The result not only confirms the existence of this ultra-rare decay but also provides a platform for further research into new physics. Karim Massri, the NA62 physics coordinator, points out the importance of this achievement:
“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.”
The Path Forward: New Physics on the Horizon?
While the current data is already groundbreaking, more data is essential to explore potential deviations from the Standard Model predictions in the K+→π+νν decay. The NA62 experiment is continuing data collection and aims to further reduce uncertainties in the coming years, potentially revealing hints of new physics.
Moreover, the technological advances made during the experiment, including detector upgrades and enhanced data-analysis tools, have enabled the NA62 team to collect signal candidates 50% faster than before. This achievement, combined with background suppression techniques, ensures that the experiment can isolate the rare K+→π+νν decay from other more common processes.
As the quest for new physics continues, the NA62 experiment’s latest discovery represents a major milestone in particle physics, offering new insights into the subatomic world and the potential for discoveries that could revolutionize our understanding of the universe.
Conclusion
The observation of the K+→π+νν decay by the NA62 experiment at CERN marks a significant step forward in the exploration of ultra-rare processes and their implications for particle physics. With this discovery, physicists are closer than ever to uncovering new physics beyond the Standard Model. As data collection continues and the search for even rarer processes advances, the NA62 experiment will remain at the forefront of efforts to push the boundaries of human knowledge about the fundamental forces of nature.
This discovery not only underscores the power of modern experimental physics but also offers hope that the next big breakthrough in understanding the universe may be just around the corner
