Record-breaking energetic particle detected under the Mediterranean is still unexplained

A telescope buried deep under the sea has caught the most energetic particle of its kind ever recorded. The international KM3NeT team announced the discovery in a scientific paper.

The particle was detected by the ARCA observatory more than two miles below the Mediterranean near Sicily.

It carried an enormous amount of energy, far greater than anything made in Earth’s particle accelerators, and points to a neutrino that likely came from outside our galaxy.

Why KM3NeT matters

Paschal Coyle, a researcher at the Marseille Particle Physics Centre at CNRS (MPPC), helps lead the KM3NeT effort.

His team operates part of the telescope from shore stations that listen to the detector through miles of cable.

“KM3NeT has begun to probe a range of energy and sensitivity where detected neutrinos may originate from extreme astrophysical phenomena,” Coyle explained.

“This first ever detection of a neutrino of hundreds of PeV opens a new chapter in neutrino astronomy and a new observational window on the Universe.” 

Deep sea telescope sees the invisible

A neutrino hardly ever hits an atom, but when it does in or near the detector, it can create a charged particle that outruns light in water and emits Cherenkov radiation.

KM3NeT’s optical modules spot that blue light, and software reconstructs the track and energy from the pattern and timing of the flashes.

ARCA is tuned for higher energies and sits far below the waves where water is dark and clear.

The event moved almost horizontally through rock and seawater for roughly 87 miles before it lit up more than a third of the photomultipliers, leaving a clean, bright signature that matched a cosmic origin.

KM3NeT telescope materials

Each digital optical module is a pressure resistant glass sphere packed with dozens of small photomultiplier tubes.

Lines of these modules rise hundreds of yards from anchors on the seafloor, with cables sending time stamped signals back to shore.

The full KM3NeT layout includes a second detector called ORCA that is placed about 1.5 miles deep off southern France.

ORCA studies how neutrinos change type as they travel, while ARCA hunts the highest energies that point back to powerful accelerators.

Keeping signals honest

Atmospheric muons rain down from above and can mimic neutrino events, but water and rock stop most of them before they reach a detector at this depth.

At the energies seen here, even a surviving atmospheric muon would not cross the long path through the seabed in this direction.

The team also considers atmospheric neutrinos, which are real neutrinos made in cosmic ray air showers, but their numbers plunge at ultra high energies.

The combination of energy, geometry, and timing points to a cosmic origin rather than a local background.

Chasing the source across the sky

Pinpointing the birthplace remains hard because the event arrived at a shallow angle and alignment uncertainties leave a patch of sky rather than a pinpoint.

The team searched for gamma ray sources and other flares in that region and did not find a clear match.

One target class is the active galactic nucleus, a galaxy with a central black hole feeding and blasting jets, but the evidence is not firm.

What the numbers tell us

The energy from this particle was far higher than anything scientists had ever measured before.

For comparison, the previous record came from IceCube, a giant neutrino observatory built into the ice at the South Pole, which recorded another neutrino at just over 6 PeV.

IceCube is made up of thousands of sensors frozen deep in Antarctic ice, designed to spot the faint flashes of light created when neutrinos interact with atoms.

In 2013, it announced the first detections of PeV-level neutrinos arriving from space.

That discovery proved that the universe produces neutrinos with enormous amounts of energy, well beyond anything humans can create on Earth.

What did KM3NeT detect?

The energy is so large that it might signal a new component in the neutrino sky at the highest energies.

One possibility raised by the KM3NeT analysis is a cosmogenic neutrino, produced when ultra energetic cosmic rays hit the diffuse photons that fill space.

Another possibility is an especially efficient accelerator in a distant galaxy. If a source class is responsible, a pattern of repeat detections with similar energies and directions should begin to appear as data accumulate.

KM3NeT is still under construction, and this detection arrived with only a fraction of the final instrument operating.

As ARCA and ORCA expand, the array will catch more events and improve its angular precision to tighten the error region on the sky.

The Mediterranean telescope works alongside IceCube at the South Pole and experiments on the surface which look for ultra high energy cosmic rays.

Together they make a network that can connect particles and photons from the same event and turn a single detection into a fuller story.

The study is published in Nature.

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