In 2020, the team reported evidence of this rare form of decay being detected by the experiment. Now, after far more collisions, including higher-energy collisions, the team reports a 5-sigma detection, meaning there is a 0.00006 percent chance that the detection is a statistical fluke.
“With this measurement, K+ → π+νṽ becomes the rarest decay established at discovery level – the famous 5 sigma,” Cristina Lazzeroni, Professor in Particle Physics at the University of Birmingham, said in a statement. “This difficult analysis is the result of excellent teamwork, and I am extremely proud of this new result.”
While the decay is rare, as predicted by the Standard Model, it is around 50 percent higher than expected, occurring about 13 times in 100 billion. It is unclear what causes this discrepancy between the Standard Model’s predictions and the results observed, with possible explanations including new particles or new physics, both of which are pretty exciting.
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