KATRIN tightens the net around the hypothetical sterile neutrino

The KATRIN (Karlsruhe Tritium Neutrino) experiment at the Karlsruhe Institute of Technology (KIT) has presented, in an article recently published in Nature, the most sensitive search for sterile neutrinos ever carried out to date. These are theoretically predicted particles which, if observed, could open the door to new physics beyond the Standard Model of elementary particles.

Neutrinos are among the most elusive particles in the universe, extremely difficult to detect and therefore still shrouded in great mystery. One of these mysteries is linked to the existence of a fourth type of neutrino, known as “sterile”. To date, in fact, we have observed only three types of neutrino, which interact with matter through the weak force: electron, muon and tau neutrinos. They are almost identical to one another, but have one single difference: when they interact with matter they produce, respectively, an electron, a muon or a tau. Along their path, neutrinos can transform, oscillating from one type to another. And since some experiments have observed an anomaly in this oscillation process, measuring the disappearance of some of these particles, the existence of sterile neutrinos has been hypothesised. These neutrinos would differ from those known so far because, for example, they would interact with matter only through the force of gravity, and not through the weak force.

Seeking to confirm the existence of these mysterious particles is precisely the objective of KATRIN experiment, whose international scientific collaboration includes 20 institutions from 7 countries, including Italy and INFN. KATRIN seeks to achieve this goal by using the beta decay of tritium, an unstable isotope of hydrogen. In beta decay, tritium emits electrons and neutrinos. In this decay, sterile neutrinos could occasionally be emitted, and their presence could be revealed by analysing the energy spectrum of these electrons.

In the study just published, the KATRIN collaboration analysed data relating to 36 million electrons observed over 259 days from 2019 to 2021 and compared them with a beta-decay model, achieving unprecedented measurement accuracy (below 1%) and demonstrating that no sterile neutrino signal was found in this data set. The result excludes a wide region of parameter space suggested by previous anomalies and claims from other experiments that had reported evidence for such a signal.

Unlike oscillation experiments, which study how neutrinos change flavour after travelling a certain distance, KATRIN probes the energy distribution at the point of creation. Based on distinct detection methods, the two approaches complement each other and together provide a powerful test that disfavors the sterile neutrino hypothesis.

“In 2026 we will upgrade the experiment by installing a new detector, TRISTAN”, explains Marco Carminati, INFN coordinator of the KATRIN experiment and professor at Milan Politecnico. “TRISTAN has been built with an extremely significant contribution from INFN and will be able to record the entire β decay spectrum of tritium with unprecedented statistics, reaching the exploration of much higher sterile neutrino masses (of the order of keV) than those investigated so far”.