In a game of snakes and ladders, players move methodically up the board, occasionally encountering opportunities to climb a ladder. The NA62 experiment at CERN is one such opportunity. Searching for ultra-rare decays at colliders and fixed- target experiments like NA62 can offer a glimpse at energy scales an order of magnitude higher than is directly accessible when creating particles in a frontier machine.
The trick is to study hadron decays that are highly suppressed by the GIM mechanism (see “Charming clues for existence“). Should massive particles beyond the Standard Model (SM) exist at the right energy scale, they could disrupt the delicate cancellations expected in the SM by making brief virtual appearances according to the limits imposed by Heisenberg’s uncertainty principle. In a recent featured article, Andrzej Buras (Technical University Munich) identified the six most promising rare decays where new physics might be discovered before the end of the decade (CERN Courier July/August 2024 p30). Among them is K+ → π+νν, the ultra-rare decay sought by NA62. In the SM, fewer than one K+in 10 billion decays this way, requiring the team to exercise meticulous attention to detail in excluding backgrounds. The collaboration has now announced that it has observed the process with 5σ significance.
“This observation is the culmination of a project that started more than a decade ago,” says spokesperson Giuseppe Ruggiero of INFN and the University of Florence. “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.”
In the NA62 experiment, kaons are produced by colliding a high-intensity proton beam from CERN’s Super Proton Synchrotron into a stationary beryllium target. Almost a billion secondary particles are produced each second. Of these, about 6% are positively charged kaons that are tagged and matched with positively charged pions from the decay K+ → π+νν, with the neutrinos escaping undetected. Upgrades to NA62 during Long Shutdown 2 increased the experiment’s signal efficiency while maintaining its sample purity, allowing the collaboration to double the expected signal of their previous measurement using new data collected between 2021 and 2022. A total of 51 events pass the stringent selection criteria, over an expected background of 18+3–2, definitely establishing the existence of this decay for the first time.
NA62 measures the branching ratio for K+ → π+νν to be 13.0+3.3–2.9 × 10–11 – the most precise measurement to date and about 50% higher than the SM prediction, though compatible with it within 1.7σ at the current level of precision. NA62’s full data set will be required to test the validity of the SM in this decay. Data taking is ongoing.
