The detector, Oscillation Research with Cosmics in the Abyss (ORCA), is part of the KM3NeT neutrino telescope array anchored to the seafloor about 1.5 miles (2.4 km) below the waves off Toulon, France. The researchers used data from 5,828 neutrinos collected over about 17 months between 2020 and 2021, looking for changes in neutrino oscillation that would signal a break in Lorentz invariance.
Lorentz invariance is a foundational assumption of both general relativity and the Standard Model of particle physics: that the laws of nature look the same no matter which way you face or how fast you move. Some theories of quantum gravity, including versions of string theory and loop quantum gravity, suggest that space-time might contain a hidden 'grain' at the tiniest scales that could violate this symmetry, but the effect would be extremely small.
To detect such a tiny effect, physicists need a probe that can travel long distances and barely interact along the way, neutrinos are nearly massless, carry zero charge and come in three types that morph into one another as they travel. If Lorentz invariance were broken, that oscillation pattern would change, with the effect growing stronger over longer distances. Some of the neutrinos studied crossed the entire 7,900-mile (12,700 km) width of Earth before being detected.
ORCA detection units are anchored to the seabed and hold 18 glass spheres packed with 31 light sensors apiece. When a neutrino collides with a particle in water, it produces a faint flash of blue light that the sensors catch. The team selected only neutrinos traveling upward through the planet, using Earth as a filter to weed out the flood of other atmospheric particles. 'Only the neutrinos can get through,' said study co-author Lukas Hennig, a doctoral student in advanced physics at Erlangen-Nürnberg.
Despite using only six detection units, a fraction of the planned 108, the team set limits on Lorentz violation that compete with those from Japan's Super-Kamiokande, which used 12 years of data, and from IceCube, which used two years from its full array buried in Antarctic ice. 'I'm surprised that we have competitive results with only six detection units,' Hennig said. The results also set the first direct limits on a kind of violation that would treat muon neutrinos and tau neutrinos differently.
'If a model predicts a Lorentz invariance violation [stronger than the new limits], the data rule it out. In that case, you can be pretty sure that this model is not correct,' Hennig said.
Today, 44 detection units are in the water, and the collaboration plans to activate 108 by 2030. The team plans to repeat the search using ORCA's sister detector, ARCA (Astroparticle Research with Cosmics in the Abyss), which sits off the coast of Sicily and is designed to catch high-energy cosmic neutrinos, where Lorentz-violating effects could be more pronounced.