How an underground water tank might hold the universe’s secrets
Neutrinos(opens in new window) are elementary particles fundamental to most processes in the universe. They are abundantly produced by the universe’s most important reactions, such as in the supernova phase of a dying star. As the probability of them interacting with matter is extremely slow, their dynamic properties don’t change over time. Consequently, they offer insights into the universe’s most important reactions. “The neutrinos released by all supernovae occurring throughout the universe’s history – referred to as the ‘diffuse supernova neutrino background’(opens in new window) (DSNB) – hold invaluable information about phenomena such as the universe’s space-time evolution, star formation, black holes and others,” explains Luis Labarga Echeverría, coordinator of the SK2HK project funded under the Marie Skłodowska-Curie Actions(opens in new window) programme.
Contributing to pioneering neutrino interactions experiments
Because of the extremely low probability of neutrino interactions occurring in detectors, the volume of material offered for interaction must be huge. Due to the expense involved in this, smart solutions have been devised, with the so-called Water-Cherenkov being the most fruitful. This technique is deployed in Japan’s Kamioka Observatory(opens in new window) by the world’s largest underground neutrino detector, the Super-Kamiokande(opens in new window) (SK) – a large tank (50 000 m3 volume) filled with ultra-pure water. When a neutrino interacts with the water, several particles are produced, with a significant number of them carrying electric charges. Around 10 000 ultra-sensitive light sensors (photomultiplier tubes, or PMTs) in the SK tank walls detect the light emitted by these charged particles travelling in water. “Reconstructing the particle’s characteristics, such as their place of origin, energy, direction and any decay processes, allows us to infer the properties of the initial neutrinos,” explains Echeverría from the Autonomous University of Madrid(opens in new window). This methodology resulted in two major achievements: the identification of neutrinos coming from the supernova SN1987 (leading to a Nobel Prize in Physics in 2002), and the discovery that neutrinos do have mass (awarded another Nobel Prize in Physics in 2015). Seconded to the SK, SK2HK’s researchers participated in further related experiments, ultimately contributing to SK observing clear indication of the DSNB, presented at the NEUTRINO 2026 conference, University of California, Irvine, June 2026 (opens in new window). “This really opens up a new era in physics, where we can investigate the evolution of the universe through the neutrinos emitted by the supernovae occurring since the dawn of time,” says Echeverría.
Towards the next-generation neutrino experiment
Better understanding neutrinos will require larger detectors, such as the Hyper-Kamiokande (HK) currently being built at the Kamioka Observatory, to which the SK2HK team have contributed. “Due to start functioning in 2028, with a volume of 260 000 tons and 20 000 light sensors, it will be significantly more efficient,” notes Echeverría. The team also participated in the research and development for new detection techniques such as using multi-PMTs instead of a single 60 cm-diameter PMT, with 19 small PMTs combined within one 60 cm-diameter case. SK2HK also lead the design and construction of the underwater casing holding the PMT sensors to withstand underwater pressures and avoid a chain reaction malfunction if one photosensor implodes. “The community is already starting to think about even larger detectors, deeper underground, with many more photosensors, which will give us even more precise information about the variables which could explain how neutrinos evolve with time, and maybe even answer the greatest riddle of all: whether the grand unification theory is correct,” concludes Echeverría.
