Physicists have detected a long-sought particle process that may suggest new forces and particles exist in the universe.

By Clara Moskowitz

Once in a very great while, an ephemeral particle called a kaon arises and then quickly decays away into three other obscure entities. Whether or not this happens in a particular way has very little bearing on most of us, who will go about our lives without knowing either way. But to physicists who have been searching for this arcane process for decades, it matters a lot; finding out how often it happens could reveal hidden aspects of our universe.

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.

Neutrinos fill the whole universe, with about 10 million of them per cubic foot, and most of them zip straight through Earth, and through particle detectors, without leaving a trace. Because they almost never interact with matter, only massive and sophisticated experiments can catch and measure the properties of neutrinos.

The subatomic particles called neutrinos are among the most elusive in the particle kingdom. Scientists have built detectors underground, underwater, and at the South Pole to measure these ghostly particles that come from the sun, from supernovae and from many other celestial objects.

In addition to measuring neutrinos from the sky, physicists on Earth use powerful accelerators to produce neutrino beams containing billions of neutrinos, of which a tiny fraction can be measured by detectors placed in the beam line. At Fermilab, the DONUT accelerator-based neutrino experiment led in 2000 to the discovery of the tau neutrino, the third of the three known types of neutrinos.

Scientists at CERN have discovered an ultra-rare particle decay process, opening a new path to find physics beyond our understanding of how the building blocks of matter interact.