Record-breaking neutrino spotted tearing through the Mediterranean Sea

A shockingly powerful neutrino that ripped through a new particle detector in the Mediterranean Sea has taken physicists by surprise, and it could be a first tantalising glimpse into some of the universe’s most cataclysmic events, such as the merging of supermassive black holes.

Neutrinos, sometimes referred to as “ghost particles”, barely interact with most matter because they are nearly massless and have no electric charge. This means that neutrino detectors typically incorporate vast amounts of dense substance, such as water or ice, in the hopes that a powerful neutrino might knock into an atom and produce a shower of particles that reveal tell-tale signs of its existence.

Damien Dornic at the Centre for Particle Physics of Marseille in France and his colleagues have done just that, spotting the most energetic neutrino ever seen. The team used the cubic kilometre neutrino telescope (KM3NeT), a pair of detector arrays at the bottom of the Mediterranean Sea, which picked up the neutrino on 13 February 2023. The detector was only 10 per cent complete at the time, so it took Dornic and his team by surprise.

“First, we were confused,” he says. “When we realised more and more that this event is truly exceptional, we were really excited.”

The signal looked promising, showing up as a nearly horizontal bright line in the detector. The researchers think this was created by small, electron-like particles called muons that were produced in the wake of the neutrino slamming through the detector and gave off light that KM3NeT’s detectors could pick up.

When the researchers first tentatively announced the result in 2024, they were still calculating the exact energy of the particle. “They were clearly surprised that they’ve seen something this high energy, so their simulations of neutrinos didn’t go that high in energy yet, they hadn’t expected to see anything this energetic,” says Morgan Wascko at the University of Oxford.

To confirm the result, the researchers had to first carefully account for the effects of other sources that could light up their detectors, such as neutrinos produced when charged particles from space, called cosmic rays, strike Earth’s atmosphere. Such signals are thought to outnumber higher energy neutrinos from more distant cosmic sources by a billion to one.

Now, they have calculated that the neutrino had an energy of 120 peta-electron volts (PeV). This is around 10 times higher than the previous record holder, discovered by the IceCube Neutrino Observatory in Antarctica. This PeV energy range is also thousands of times more than the most energetic particles produced at accelerators on Earth, such as the Large Hadron Collider at CERN.

Detecting such high-energy neutrinos can give us unique insights into the events that produce them, such as black holes accreting matter or supernova explosions, which themselves give off cosmic rays that produce neutrinos as they are made. “Cosmic rays are charged, and we lose most of their original formation location as they traverse interstellar space, but the neutrinos will point straight back,” says Wascko.

Dornic says that in this case, following the neutrino back leads to a relatively large patch of space, making it difficult to locate an exact source, but planned improvements to the telescope should allow them to pinpoint an object should a similarly powerful neutrino be spotted in future.