August 1998: Scientists publish evidence that neutrinos oscillate

More than 3,000 feet beneath a forested peak in Japan’s Northern Alps sits a 13-story, stainless steel cylindrical tank filled with 13 million gallons of ultra-pure water. This behemoth buried under Mount Ikeno, known as Super-Kamiokande, or “Super-K,” is lined with 13,000 golden, bulb-like light sensors that capture signals of the lightest particles known to humanity: neutrinos.

When the detector began operating in 1996, neutrinos were still thought to be massless. But just two years later, Super-K would capture neutrinos changing “flavor,” or type — upending decades-long assumptions about the nature of these elusive particles and definitively confirming that they do, in fact, have mass.

Nicknamed “ghost particles,” neutrinos are notoriously hard to pinpoint. When Wolfgang Pauli postulated their existence in 1930 to explain why energy went missing during nuclear beta decay, he lamented that the particles could never be detected. Four years later, Enrico Fermi, knowing that the particles are chargeless and have little or no mass, devised the theory behind the weak nuclear force that governs their rare interactions with matter. But it wasn’t until 1956 that physicists finally captured neutrinos by placing an underground detector near a nuclear plant, where nuclear fission provided a potent source of antineutrinos.

But the reactor experiment only scratched the surface. Earth is constantly being showered with neutrinos from extraterrestrial sources: the sun, cosmic rays, and even supernovae from the early universe. Trillions of these subatomic particles stream through our bodies every second. However, because they interact so weakly with matter, detecting even a handful requires a vast space and a shield from cosmic noise. Scientists began carving out enormous caverns deep underground to house giant detectors that could spot these phantom particles.

In 1968, the Homestake experiment used a 100,000-gallon vat of dry-cleaning fluid buried in a gold mine nearly a mile beneath South Dakota to capture neutrinos from the sun. The experiment proved that our star is powered by nuclear fusion and that solar neutrinos — generated as electron neutrinos during fusion processes in the sun’s core — were reaching Earth. But it caught less than half of what theory predicted, hinting that these neutrinos could be “oscillating” en route to Earth, shapeshifting into undetected flavors. Although neutrino oscillations had been hypothesized more than a decade earlier, there was still no experimental proof. Homestake’s solar neutrino anomaly would remain unsolved for the next three decades.

By the 1980s, the Kamioka Observatory, once a zinc and lead mine beneath the Hida Mountains, had become a major site for neutrino discoveries. In 1985, Super-K’s predecessor, a particle detector originally designed to search for proton decay, began recording solar and atmospheric neutrinos. The latter are a mixture of electron and muon neutrinos created when high-energy cosmic rays collide with nuclei in Earth's upper atmosphere. But like the Homestake experiment, the Kamiokande detector’s numbers were off. The measurements were missing roughly half of the muon neutrinos predicted by theory.

Scientists had observed atmospheric neutrinos in two deep gold mines 20 years earlier, but it wasn’t until detectors like Kamiokande and IMB came online that researchers could achieve the scale, resolution, and particle identification capabilities necessary to see the anomaly. Takaaki Kajita, a young scientist working on the original Kamiokande experiment, was baffled by the results. After completing his doctoral studies in 1986, Kajita and his colleagues set about trying to find mistakes in the data. Ultimately, no errors or uncertainties could explain the anomaly, and, while the case for neutrino oscillations continued to grow, there was still no definitive proof.

Like its predecessor, Super-K is a water Cherenkov detector — a giant water tank outfitted with ultra-sensitive light sensors called photomultiplier tubes. When neutrinos pass through, they rarely but occasionally collide with water molecules. These collisions emit cones of faint blue light, called Cherenkov radiation, that project as rings onto the sensor-lined walls, helping determine the neutrinos’ type and direction. With 16 times the water capacity and 10 times more photosensors than its predecessor, Super-K could capture more neutrino events with much higher precision. The detector was set to resolve the neutrino anomaly once and for all, and Kajita was tasked with leading the effort.

After sifting through thousands of atmospheric neutrino signals in the first two years of Super-K experiments, Kajita’s team found that muon neutrinos arriving from the atmosphere above were much more abundant than muon neutrinos from Earth’s interior below. The abundance of electron neutrinos, however, remained unchanged. These measurements finally gave them evidence to deduce that neutrinos changed flavor as they traveled. They published their findings in Physical Review Letters in August 1998.

Confirmation came a few years later. In 2000, scientists proved the existence of the last known neutrino flavor: tau. A year later, the Sudbury Neutrino Observatory, or SNO, in Canada — a Cherenkov detector filled with heavy water — was able to isolate solar neutrinos from the total neutrinos by measuring how they collided with deuterons. It showed that two-thirds of solar neutrinos went missing while the total neutrinos remained the same — confirming that neutrinos were oscillating. These oscillations meant that neutrinos possess mass and that their underlying mass states differ — a direct consequence of quantum mechanics. Takaaki, along with Arthur McDonald from SNO, went on to win the 2015 Nobel Prize in physics for their discoveries.

New physics is required to explain how neutrinos get their mass, tiny as it may be. The Standard Model requires that a particle and its antiparticle, if they have mass, must both have right-handed and left-handed versions — yet only left-handed neutrinos and right-handed antineutrinos have ever been observed. Other questions also remain unanswered: How do the masses of the different neutrino types compare to one another? Do neutrinos and antineutrinos oscillate at different rates, and could it help explain the universe’s matter-antimatter asymmetry? Next-generation projects like Hyper-Kamiokande and DUNE aim to answer these questions. But for now, Super-K continues to investigate these chameleon-like particles.