KATRIN Experiment Tightens the Net Around the Elusive Sterile Neutrino
Neutrinos are some of the most mysterious particles in the universe. They are everywhere—streaming through your body by the trillions every second—yet they barely interact with anything at all. For decades, physicists believed neutrinos were massless, but discoveries in the late 20th century revealed that they do have mass and can even change their identity as they travel. This breakthrough reshaped particle physics and opened the door to an even more intriguing possibility: the existence of a fourth type of neutrino, known as the sterile neutrino.
A major new result from the KATRIN experiment now delivers the most precise test yet of this idea. After years of extremely sensitive measurements, scientists report no evidence for sterile neutrinos—while sharply narrowing where such particles could still be hiding.
What Is the Sterile Neutrino and Why Does It Matter?
The Standard Model of particle physics includes three types of neutrinos: electron, muon, and tau neutrinos. These particles interact via the weak nuclear force and gravity, making them notoriously hard to detect. Sterile neutrinos, if they exist, would be even more elusive. They would not interact via the weak force at all—only through gravity and subtle mixing effects with ordinary neutrinos.
The idea of sterile neutrinos did not arise from theory alone. Over the past few decades, several experiments—particularly reactor neutrino experiments and gallium source experiments—have reported anomalies. These were small but persistent discrepancies between predicted and observed neutrino counts. One explanation was that some neutrinos were “disappearing” by transforming into sterile neutrinos.
If confirmed, sterile neutrinos would be new fundamental particles, lying beyond the Standard Model and potentially linked to dark matter. This is why the question has remained one of the most important unsolved problems in neutrino physics.
How the KATRIN Experiment Searches for Sterile Neutrinos
The Karlsruhe Tritium Neutrino (KATRIN) experiment, located at the Karlsruhe Institute of Technology (KIT) in Germany, was originally designed to answer a different question: How much do neutrinos weigh? To do this, KATRIN studies the beta decay of tritium, a radioactive form of hydrogen.
When tritium decays, it emits an electron and a neutrino. By measuring the energy spectrum of the emitted electrons with extraordinary precision, scientists can infer properties of the neutrino. Even tiny changes in the spectrum carry important clues.
If a sterile neutrino exists, it would occasionally be produced during tritium beta decay. This would leave behind a distinctive feature in the electron energy spectrum—a small distortion or “kink” at a specific energy. Detecting such a kink would be a clear, direct signature of a sterile neutrino.
What makes KATRIN especially powerful is its scale and precision. The experiment stretches over more than 70 meters and includes three major components:
- A windowless gaseous tritium source that produces a steady stream of electrons
- A sophisticated transport and pumping system that guides electrons while removing tritium gas
- A massive electrostatic spectrometer that measures electron energies with unmatched accuracy, followed by a detector that counts the electrons
Unlike oscillation experiments, which study how neutrinos change flavor as they travel, KATRIN probes neutrino properties right at the moment of creation.
The New Results Published in Nature
In a new study published in Nature, the KATRIN collaboration presents the most sensitive direct search for sterile neutrinos to date using tritium beta decay.
The analysis is based on 259 days of data collected between 2019 and 2021, during which KATRIN recorded around 36 million electrons. The measurements reached sub-percent accuracy, an extraordinary level of precision for such an experiment.
After comparing the observed electron energy spectrum with detailed theoretical models, the team found no sign of the characteristic kink that would indicate the presence of a sterile neutrino.
This null result is not a disappointment—it is a powerful scientific statement.
What These Findings Rule Out
The new KATRIN data exclude a large region of sterile neutrino parameter space, particularly in the mass range from a few to several hundred electron volts squared (eV²). This directly challenges earlier hints from reactor and gallium experiments that suggested the presence of a light sterile neutrino.
Most notably, the results fully rule out the claim made by the Neutrino-4 experiment, which had previously reported evidence for sterile neutrinos with specific mass and mixing parameters. KATRIN’s measurements show that such a signal should have been clearly visible—but it is not there.
The experiment’s excellent signal-to-background ratio plays a crucial role here. Nearly all detected electrons originate from tritium beta decay, making the measurement exceptionally clean and reliable.
How KATRIN Complements Other Neutrino Experiments
Sterile neutrino searches rely on different experimental strategies, and this is where KATRIN’s strength becomes especially clear.
Reactor experiments like STEREO are most sensitive to sterile neutrinos with small mass splittings, typically below a few eV². KATRIN, on the other hand, probes a higher mass range, extending up to several hundred eV².
Together, these approaches now paint a consistent picture: light sterile neutrinos that significantly mix with known neutrino types are strongly disfavored. Instead of conflicting results, the field is converging toward tighter and more unified constraints.
What Comes Next for the KATRIN Experiment
KATRIN is far from finished. Data collection will continue through 2025, and by the end of this phase, the experiment is expected to record more than 220 million electrons in the region of interest. This represents an increase in statistics by over a factor of six, allowing scientists to push sensitivity to even smaller mixing angles.
Beyond that, an exciting upgrade is already planned. In 2026, KATRIN will be equipped with a new detector system called TRISTAN. Unlike the current setup, TRISTAN will measure the entire tritium beta-decay spectrum directly, bypassing the main spectrometer.
This upgrade will allow KATRIN to explore much heavier sterile neutrinos, reaching into the keV mass range—a region of particular interest because sterile neutrinos at these masses are potential dark matter candidates.
Why Sterile Neutrinos Remain an Open Question
Even though KATRIN’s latest results significantly narrow the possibilities, they do not completely close the case on sterile neutrinos. Heavier sterile neutrinos, ultra-weakly mixed states, or particles with different properties could still exist beyond current experimental reach.
What the new findings do achieve is clarity. They eliminate several long-standing explanations for earlier anomalies and provide one of the cleanest experimental tests of sterile neutrino models ever performed.
In the broader context of particle physics, this is how progress is made—not only by discoveries, but also by precise exclusions that guide future theories and experiments.
Research Paper:
https://www.nature.com/articles/s41586-025-09739-9
