High-energy cosmic neutrino detected under Mediterranean Sea

Using an observatory under construction deep beneath the Mediterranean Sea near Sicily, scientists have detected a ghostly subatomic particle called a neutrino boasting record-breaking energy in another important step toward understanding some of the universe's most cataclysmic events.

The researchers, part of the KM3NeT (Cubic Kilometre Neutrino Telescope) Collaboration, believe the neutrino came from beyond the Milky Way galaxy. They identified 12 supermassive black holes actively guzzling surrounding matter at the centre of distant galaxies as possible origination points, though the neutrino may have arisen from some other source.

KM3NeT comprises two large neutrino detectors at the bottom of the Mediterranean. One called ARCA - 3,450 meters (2.1 miles) deep near Sicily - is designed to find high-energy neutrinos. One called ORCA - 2,450 meters (1.5 miles) deep near Provence, France - is designed to detect low-energy neutrinos.

The newly described "ultra-high-energy" neutrino, detected by ARCA in February 2023, was measured at about 120 quadrillion electronvolts, a unit of energy.

It was 30 times more energetic than any other neutrino detected to date, a quadrillion times more energetic than particles of light called photons and 10,000 times more energetic than particles made by the world's largest and most powerful particle accelerator, the Large Hadron Collider near Geneva.

"It's in a completely unexplored region of energy," said physicist Paschal Coyle of the Marseille Particle Physics Centre (CPPM) in France, one of the leaders of the research published on Wednesday in the journal Nature.

"The energy of this neutrino is exceptional," added physicist Aart Heijboer of the Nikhef National Institute for Subatomic Physics in the Netherlands, another of the researchers.

Neutrinos offer scientists a different way to study the cosmos, not based on electromagnetic radiation - light. Many aspects of the universe are indecipherable using light alone.

Neutrinos are electrically neutral, undisturbed by even the strongest magnetic field, and rarely interact with matter. As neutrinos travel through space, they pass unimpeded through matter - stars, planets or anything else.

That makes them "cosmic messengers" because scientists can trace them back to their source, either within the Milky Way or across galaxies, and thus learn about some of the most energetic processes in the cosmos.

"Neutrinos are ghost particles. They travel through walls, all the way through the Earth, and all the way from the edge of the universe," Coyle said. "Neutrinos have zero charge, zero size, almost zero mass and almost zero interaction. They are the closest thing to nothing one can imagine, but nevertheless they are key to fully understanding the universe."

Other high-energy cosmic messengers zipping through space are not as reliable. For instance, the path of cosmic rays gets bent by magnetic fields, so they cannot be traced back to their place of origination.

Detecting neutrinos is not simple, requiring large observatories located deep underwater or in ice. These mediums offer an expansive and transparent volume where a passing neutrino may interact with a particle, producing a flash of light called Cherenkov radiation.

The researchers concluded that the one spotted at ARCA - which was a type of neutrino called a muon - was of cosmic origin based on its horizontal trajectory and the fact that it had traversed through about 140 km (87 miles) of rock and seawater before reaching the detector.

The KM3NeT detectors are still under construction and have not yet reached their full capabilities.

Neutrinos are produced through various astrophysical processes at various energy levels. For instance, low-energy neutrinos are born in nuclear fusion processes inside stars.

High-energy neutrinos arise from particle collisions occurring in violent events such as a black hole greedily eating infalling matter or bursts of gamma rays during the explosive deaths of stars. They also can be produced by interactions between high-energy cosmic rays and the universe's background radiation.

The study of neutrinos is still in its formative stages.

"So why it matters? It's basically just trying to understand what is going on in the cosmos," Heijboer said.