Neutrinos are everywhere, but difficult to detect. Because these almost massless elementary particles hardly interact with matter. They arise from radioactive nuclear decay, from nuclear fusion in stars, but also from high-energy cosmic explosions, such as the supernova of a star. A single such core collapse supernova can contain the enormous amount of 1058 Release neutrinos of all kinds.
Search for ancient supernova neutrinos
Astrophysicists suspect that the entire universe is filled with a diffuse supernova neutrino background (DSNB). This cosmic background noise would contain neutrinos from almost all stellar explosions since the Big Bang – and could therefore reveal a lot about the history of our universe. According to models, these supernova neutrinos, some of which are ancient, still have energies between one and a few dozen megaelectron volts.
Since the Big Bang, stellar explosions have repeatedly released neutrinos. These should still be detectable today as a diffuse supernova neutrino background. — © Kamioka Observatory/ Institute for Cosmic Ray Research, The University of Tokyo
But this is exactly what makes it difficult to prove the neutrino background, which has so far only been postulated in theory. In this energy range, neutrinos from nearby sources such as the sun, radioactive decay in earth’s rocks or from nuclear reactors overshadow the weak “whisper” of supernova neutrinos. Only a few neutrino detectors worldwide are sufficiently shielded and equipped to even be able to identify this signal.
Flashes of light in the underground tank
One of these neutrino detectors is the Super-Kamiokande in Gifu Prefecture, Japan. This underground observatory, located under a thousand meters of rock, uses a tank with 50,000 liters of ultra-pure water as a search aid. When a neutrino collides with a water molecule in this tank, secondary particles and a weak flash of light are produced, which are registered by around 13,000 photosensors in the walls of the detector tank.
A team led by Hiroyuki Sekiya from the Super Kamiokande Collaboration has now evaluated data from 5,000 days of observation of this neutrino detector to search for traces of the supernova neutrino background. The data comes partly from measurements with pure water in the detector tank and partly from an improved, more sensitive measuring phase in which gadolinium was added to the water.
Super Kamiokande detector in Japan detects neutrinos through their collisions with water protons and gadolinium atoms and the secondary particles and flashes of light that are produced. — © Kamioka Observatory/ Institute for Cosmic Ray Research, The University of Tokyo
Telltale neutrino excess
And indeed: The physicists discovered a statistically significant signal – a slight excess of neutrino signals in the energy range between 13.3 and 81.3 megaelectron volts. This excess is in a range that fits the supernova background, as the team explains. The detected signal corresponds to an influx of an average of 3.6 additional neutrinos per square centimeter per second – too much to be explained by mere coincidence.
However: With a significance of 2.6 sigma, the neutrino excess detected at the Super-Kamiokande is not enough to be officially considered proof – a significance of 5 sigma would be required for this. The physicists therefore describe their – still preliminary – results as strong evidence: “Super-Kamiokande has detected the first indication of the diffuse supernova neutrino background,” they write.
Preliminary results of the search for the diffuse supernova neutrino background with Super-Kamiokande. The red area indicates the neutrino excess. — © Super-Kamiokande Collaboration
However, Sekiya and his team are confident that they can increase the significance of their results with further measurements. They already see their signal as an important breakthrough: “Observing the world’s first indication of the diffuse supernova neutrino background is a deeply significant achievement,” says Sekiya. “This has been a long-held goal since the beginning of the Super Kamiokande project.”
Source: Hiroyuki Sekiya (University of Tokyo) et al., Neutrino 2026: XXXII International Conference on Neutrino Physics and Astrophysics, presentation