Neutrinos are invisible, difficult to measure and still pose a mystery – for example, whether there is a fourth type in addition to the three known ones, the “sterile neutrinos”. Now new data from the KATRIN experiment in Karlsruhe make this hypothetical fourth type of neutrino very unlikely. Because if they exist, the sterile neutrinos should leave a measurable “bent” in the energy spectrum of electrons from tritium decays. After evaluating 36 million electron spectra, the physicists in the KATRIN collaboration found no evidence of such a signal, as they now report. This excludes most of the energy and mass range for a fourth type of neutrino. This makes the existence of such particles that were not previously included in the Standard Model much less likely.
Neutrinos are the most common particles of matter in the universe; billions of them race through us every second – without us noticing. Neutrinos have almost no mass and only interact with other particles via the weak nuclear force. This makes them difficult to detect. To make matters worse, neutrinos come in three types – physically referred to as “flavors”: electron, muon and tau neutrinos. These can literally transform into one another in flight. At the same time, this neutrino oscillation means that the neutrinos, which were originally assumed to be massless, must have a mass, albeit a very small one. But in recent decades there has been increasing evidence that there could be a fourth type of neutrino. These so-called “sterile” neutrinos are said to interact even weaker or not at all with matter and other particles and are therefore not directly detectable. However, if they exist, they could explain a whole range of previously unexplained phenomena – from dark matter particles to the asymmetry of antimatter and matter.
Neutrino “Libra” is looking for the fourth variety
But so far it is controversial whether this hypothetical fourth type of neutrino exists. Anomalies that occurred in some experiments measuring neutrinos and their oscillation provided evidence for their existence. There were slight deviations in the expected number of neutrinos detected – for example from radioactive decay in nuclear reactors or in radiochemical detection through gallium decay. But other experiments, such as the IceCube neutrino detector at the South Pole or the STEREO experiment at a research reactor in Grenoble, failed to detect such deviations. “The existence of sterile neutrinos remains controversial; this is primarily due to the challenge of fully understanding the systematic measurement uncertainties and background of each experiment,” explain the physicists of the KATRIN collaboration. They have therefore now used data from the KATRIN experiment (Karlsruhe Tritium Neutrino) at the Karlsruhe Institute of Technology to search for traces of the fourth type of neutrino.
In the 70 meter long KATRIN facility, highly sensitive spectrometers measure the beta decay of radioactive tritium. This releases an electron and an antineutrino. The energy of the electron reveals how much energy and mass the neutrino has. Actually, KATRIN is used to determine the mass of the neutrino as accurately as possible. In the spring of 2025, physicists managed to limit this quantity, which is important for physics and cosmology, to less than 0.45 electron volts. But the highly precise measurements of the energy spectrum of the emitted electrons can also reveal whether there are sterile neutrinos among the neutrinos released during decay. “Such a sterile neutrino would leave a double trace in the observed electron spectrum: a noticeable kink and a general distortion,” explain the physicists in the KATRIN collaboration. For their current study, they evaluated the energy spectra of more than 36 million electrons, which were detected and measured over the course of 259 days of measurement. The KATRIN experiment covers an energy and mass range of the hypothetical particles from less than one electron volt to several hundred electron volts.
No significant signal found
The result: “No significant signal from sterile neutrinos was found in the KATRIN search,” reports the collaboration. Neither a noticeable kink in the energy spectrum of the electrons nor a broader distortion of the spectrum could be detected. Because the KATRIN experiment measures the energies of the released electrons with sub-electron volt precision and very low background noise, the reliability of these results is high, say the physicists. Their results contradict previous measurements in reactor neutrino and gallium source experiments, but at the same time they are based on a different measurement method. Because KATRIN measures the energy distribution directly at the point of origin, not after a flight path with neutrino oscillation like many other detectors. This makes the KATRIN data suitable as a supplement to other measurement approaches and thus provides a meaningful test for sterile neutrinos. “Our new result fully complements reactor experiments like STEREO,” says co-author Thierry Lasserre from the Max Planck Institute for Nuclear Physics in Heidelberg.
Specifically, the new results of the KATRIN experiment now exclude large parts of the energy and mass range in which the sterile neutrinos could possibly hide. They completely refute the parameter space determined in the Neutrino 4 experiment and exclude most of the region in which reactor neutrino and gallium source experiments had found possible evidence of the fourth type of neutrino, as the physicists in the KATRIN collaboration report. The results of the STEREO and KATRIN experiments together now consistently rule out light sterile neutrinos, explains Lasserre. This means that the space in which sterile neutrinos could still hide is significantly smaller and the probability of their existence has fallen sharply.
In the future, the KATRIN experiment could set even stricter limits for the controversial particles. Because the measurements continue. “By the time data collection is completed in 2025, KATRIN will have recorded more than 220 million electrons in the relevant area, increasing the statistics more than sixfold,” says KATRIN co-spokeswoman Kathrin Valerius from the Karlsruhe Institute of Technology. In 2026, the KATRIN experiment will also be upgraded with an additional detector, which can extend KATRIN’s range to larger masses of sterile neutrinos. “We will thus open a new window into the kiloelectronvolt mass range. In this range, sterile neutrinos could even represent a building block of dark matter in the universe,” says co-spokeswoman Susanne Mertens, director at the Max Planck Institute for Nuclear Physics.
Source: KATRIN Collaboration, Nature, doi: 10.1038/s41586-025-09739-9