A Rare Kaon Decay Could Hold the Key to New Physics

In a groundbreaking experiment at CERN, particle physicists are on the brink of uncovering new physics beyond the Standard Model. The NA62 experiment, led by a team of researchers including Cristina Lazzeroni from the University of Birmingham, has successfully observed and measured a rare particle decay event that scientists have long sought to witness. The event? The decay of a charged kaon into a charged pion, a neutrino, and an antineutrino—a channel so rare that it has earned the status of a "golden" event in particle physics.
For more than a decade, the NA62 team has been laser-focused on this specific type of decay. What makes it so special is not only its extreme rarity but also the fact that it is precisely predicted by the Standard Model—the current best explanation of how the fundamental particles and forces in the universe behave. But with particle physics, rarity is what makes certain decay processes invaluable for probing new physics. Even the smallest deviation from the predictions of the Standard Model could open doors to unknown realms of physics.
This rare kaon decay into a pion and a neutrino-antineutrino pair, occurring only in about 13 out of 100 billion kaon decays, provides a sensitive test for new physics. Yet, it has taken years of data collection and analysis, with physicists sorting through countless particle collisions, to confirm this event with the utmost precision. As Lazzeroni remarked, this milestone in experimental physics is the result of a remarkable team effort and years of dedication.

Kaons: A Window into the Fundamental Forces
Kaons, the particles at the heart of this experiment, are a fascinating class of subatomic particles. They consist of a quark bound with an antiquark and belong to a family known for their strange behavior—literally. Kaons exhibit "strangeness," a unique property that makes them invaluable in understanding how different types, or "flavors," of quarks behave under the strong force.
The NA62 experiment produces kaons by firing high-energy protons at a beryllium target, creating a secondary beam of particles. Around 6% of these particles are kaons, but these particles are fleeting, existing only for a mere hundred-millionth of a second before decaying. Most kaon decays result in a muon and a neutrino, but in this incredibly rare event, the kaon decays into a pion, neutrino, and antineutrino—a process mediated by the Z boson, one of the carriers of the weak force.
This decay is exceptionally challenging to detect. The experiment doesn't even try to capture the elusive neutrinos or antineutrinos, which annihilate almost immediately after being produced. Instead, it focuses on spotting the pion, a charged particle, as a telltale sign that this rare kaon decay has occurred.

The Path to New Physics
The rarity and precision required to measure this decay channel make it a powerful tool for testing the Standard Model. The current results from NA62 show a number of decays slightly higher than the Standard Model prediction of 8.4 per 100 billion, but still within acceptable uncertainty limits. For now, these results are consistent with the Standard Model. However, as Lazzeroni explains, the Standard Model, despite its successes, does not account for several cosmic mysteries, such as dark matter or the matter-antimatter imbalance in the universe.
The hope is that as the NA62 team gathers more data over the next three years, they will find deviations from the Standard Model’s predictions. Even a small discrepancy could signal the presence of new particles or forces that might fundamentally alter our understanding of the universe.
Lazzeroni and her team are optimistic. "The Standard Model has been very good at predicting observations so far, but we know it must have shortcomings," she said. "We expect new physics to appear—what it is exactly, it's unknown. But generally speaking, we expect new particles and forces to be present."
The results from NA62 could one day provide critical clues for solving some of the universe's greatest mysteries. While no new physics has emerged just yet, the fact that the NA62 experiment is probing these uncharted territories makes the future incredibly promising. With more data in hand, the team may soon reveal whether this rare decay points toward the discovery of new forces and particles that could reshape modern physics.
The team recently presented these findings at a CERN seminar, marking a critical milestone in their search for the extraordinary in the ordinary decay of kaons. The next few years could prove pivotal as they collect even more data and inch closer to the possibility of new physics.

A New Chapter in Particle Physics?
This achievement by Lazzeroni’s team is a significant step in particle physics, offering a new frontier for understanding the universe. With ongoing experiments and continuous data collection, scientists may be on the verge of discovering phenomena that could challenge our deepest scientific assumptions.
The tantalizing possibility of finding "new physics" has energized the field, and CERN’s NA62 experiment is now firmly at the forefront of that quest. How wonderfully exciting!