n 19.03.25 – EPFL researchers, working with the Centre de Physique des Particules de Marseille, have developed a new method for identifying neutrinos using data from CERN’s NA62 experiment—which recently observed the rarest particle decay ever recorded.
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Particle physics explores the smallest building blocks of nature, particles so tiny that trillions of them pass through us unnoticed every second. Among them are the kaons, particles made from a quark and an antiquark. Kaons are fascinating because they undergo “decay”—a process where unstable particles transform into more stable ones, shedding energy in the process. By studying these decays, physicists probe the fundamental forces and test the Standard Model, the framework that explains how particles and forces interact.

Kaons are especially important in “flavor physics,” a field that studies how different types, or “flavors,” of quarks interact and transform. Rare decays, like a kaon transforming into a pion and a neutrino-antineutrino pair, reveal intricate details of these interactions. This decay, called the “golden channel” of flavor physics, happens only once in ten billion kaon decays. Nonetheless, it can reveal key information about the weak force and the universe’s fundamental symmetries.

Milestone: observing a rare kaon decay

Since 2023, EPFL has extended its physics research expertise to research with kaons, becoming the first Swiss institution part of CERN’s NA62 experiment. In September 2024, NA62 reported the first observation of the rare kaon decay, transforming into a pion and a neutrino-antineutrino pair.

The observation was the result of analyzing a massive amount of experimental data from 2016 to 2022, and advanced technology to isolate the decay. And while the result aligns with the Standard Model of physics, the decay rate is 50% higher than predicted, hinting at potential discoveries beyond current physics.

Radoslav Marchevski, Professor at EPFL’s High Energy Physics Laboratory since 2023, and his team have significantly contributed to the data analysis that led to the observation. “Over the past decade, we’ve contributed substantially, from demonstrating NA62’s capability to measure this process and continuing the effort to improve the measurement to proposing setup modifications in 2021 that improved sensitivity,” he says.

mu^+ \nu_\mu decay. The tagged neutrino interacts in the electromagnetic calorimeter filled with Liquid Krypton (LKr) of NA62 to produce a hadron shower, which produces a clusters of energy deposition in the LKr and develops further in the hadron calorimeters (MUV1 + MUV2). The second muon produced in the neutrino interaction is also detected by the muon veto system (MUV3) compatible with the direction of the neutrino. The charged hodoscope (CHOD), detecting the position of charged particles, sees only one muon as the neutrino does not leave a signal there. Credit: NA62 collaboration” height=”1312″ src=”//actu.epfl.ch/public/upload/fckeditorimage/fb/f2/6ead15b9.jpg” width=”1729″/>

A new technique for neutrino tagging

But the contribution of EPFL didn’t stop there. To improve the detection capabilties of the NA62 experiment, Marchevski’s team worked with the Centre de Physique des Particules de Marseille (CPPM) to pioneer a groundbreaking “neutrino-tagging technique”, a method used to identify neutrinos and link them to the particle events that produced them.

What are neutrinos? They are some of the most mysterious particles, interacting so weakly that billions pass through the Earth without leaving a trace. They come in three “flavors” — electron, muon, and tau — corresponding to the type of charged particle they are associated with during interactions. Identifying the flavor of a neutrino is crucial for studying phenomena like neutrino oscillations, where neutrinos switch flavors as they travel.

The new method developed by EPFL and CPPM uses kaon decays to tag the neutrino flavor on an event-by-event basis, which has never been done before. This is groundbreaking because it directly ties each neutrino to its parent particle, enabling precision measurements of neutrino properties.

“This novel technique allows us to tag neutrino flavors as they are produced and to precisely associate them to interactions in the active area of our detectors, something that has never been done before,” explains Marchevski. “It opens unique opportunities for future neutrino physics experiments by providing much better energy resolution and neutrino flavor tagging, which will enable precision studies of neutrino properties.”

Both discoveries are important for physics: Observing the rare kaon decay points to some tension with the Standard Model, and the possibility of “new physics” – phenomena that go beyond our current understanding of the subatomic world. Meanwhile, the neutrino-tagging technique could transform future experiments, offering tools to probe neutrino interactions and their fundamental role in the cosmos.

Marchevski adds: “With EPFL now part of NA62, we’ve brought kaon physics to Switzerland, leading key contributions, including a novel neutrino-tagging technique, and showcasing EPFL’s growing visibility in international particle physics.”

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