Neutrinos: they have no electric charge, pass through matter like a ghost, and are so light they were initially thought to have zero mass. These are just some of the traits that make them so difficult to detect. Research on neutrinos requires massive underground observatories far away from potential confounders that drown out their weak signals. One of the largest in the world, located 1,000 meters underground in Gifu Prefecture, Japan, is called the Super-Kamiokande.
For the first time ever, the Super-Kamiokande Collaboration found an indication of the Diffuse Supernova Neutrino Background (DSNB), which is an integrated flux of neutrinos originating from many different supernovae over time. This international collaboration group involving approximately 250 researchers from 60 universities and research institutions has made a ground-breaking achievement that provides an important clue for deepening our understanding of the history of cosmic star formation and nucleosynthesis.
The research results were presented on June 25, 2026, at Neutrino 2026: XXXII International Conference on Neutrino Physics and Astrophysics, held at the University of California, Irvine, USA.
The DSNB is the accumulation of neutrinos emitted by all core-collapse supernovae throughout cosmic history, from the early universe to the present. Capturing the DSNB would provide a definitive observational means to quantitatively unravel the history of nucleosynthesis and star formation in the universe, and to test theoretical models. However, neutrinos arriving from vast distances are diffuse, and their signals are extremely faint and challenging to detect. Undertaking this observation is like straining to hear the "faint whispers" of supernova explosions engraved in cosmic history.
To tune in to these "whispers", the research team conducted a detailed analysis of approximately 5,000 days of observational data, combining two phases of data collection involving either ultrapure water or ultrapure water with the addition of Gadolinium (which improves detection). Super-Kamiokande detects Cherenkov light produced when neutrinos interact with water, using a 50,000-ton tank of ultrapure water and approximately 13,000 photomultiplier tubes installed underground. This level of dedication is required just to be able to potentially detect neutrinos and minimize background noise such as cosmic rays.
Ultimately, the team identified a statistically significant excess signal in the neutrino energy range from 13.3 to 81.3 MeV. The significance of the excess signal was 2.6 sigma (99.5% confidence level). Although it cannot be explained as a random fluctuation, it does not yet meet the discovery threshold (5 sigma or higher) and is therefore currently described as an indication rather than a definitive detection.
"We are already planning on incorporating ongoing observations at Super-Kamiokande together with its successor detector, Hyper-Kamiokande, to further improve sensitivity in future collaborative studies," says Yosuke Ashida, Assistant Professor at Tohoku University.
The current results are anticipated to contribute to a better understanding of the formation processes of neutron stars and black holes, as well as the chemical evolution of the universe.
Regarding this result, Hiroyuki Sekiya, Associate Professor at the University of Tokyo, and spokesperson for the Super-Kamiokande experiment, commented: "Observing the world's first indication of the Diffuse Supernova Neutrino Background is a deeply meaningful achievement and has been a long-cherished goal since the beginning of the Super-Kamiokande project."