'Cosmic Messenger': Mysterious 'Very High Energy Neutrino' Detected From Outside Our Galaxy

A galaxy far, far away. The CNRS speaks of an "earthquake" and a "new window on the Universe". A neutrino (a particle very present in the Universe, Editor's note) with an energy thirty times higher than any ever recorded on Earth has been detected at the bottom of the Mediterranean , we learn in a study published this Wednesday, February 12 in the journal Nature .

It is the most energetic of elementary particles. And it should help to make a significant advance in the understanding of extreme phenomena in the Universe.

This discovery "shakes up current astrophysical models", writes the CNRS in a press release.

Although the origin of this incredible particle has not yet been identified, scientists are certain that it does not come from our galaxy.

An elementary particle abundant in the Universe but elusive, the neutrino has, as its name indicates, no electric charge, and almost no mass: the latter is a million times weaker than that of an electron. It is also the lightest of the known massive particles. In addition, it interacts only weakly with matter.

Neutrinos are of particular interest to scientists because they are "special cosmic messengers," explains Rosa Coniglione, a researcher at the Italian Institute of Nuclear Physics, in a press release accompanying the publication of the study.

The most violent events in the Universe - such as a supernova explosion, the merger of two neutron stars or the activity around supermassive black holes - generate so-called "ultra-high energy" neutrinos.

Since these particles interact little with matter, they can escape from the dense and turbulent zones that produced them, then travel in a straight line across the Universe. And thus provide valuable information, inaccessible by more classical methods, on the astrophysical phenomena at their origin.

"For illustration, if we wanted to stop half of the neutrinos coming towards us, we would have to build a lead wall nine thousand billion kilometers thick," explains Sonia El Hedri, an astrophysicist at the CNRS, in a video .

These "ghost" particles are, however, extremely difficult to detect. 60 billion neutrinos pass through each square centimeter of the Earth per second without leaving the slightest trace, specifies the CNRS.

To hope to catch a few in flight, you need a huge volume of water - at least one cubic kilometer, the equivalent of 400,000 Olympic swimming pools. That's why the Mediterranean is home to the Cubic Kilometer Neutrino Telescope (KM3NeT).

Still under construction, it is spread over two sites: ARCA, dedicated to high-energy astronomy, at a depth of 3,450 meters off the coast of Sicily (Italy) and ORCA, optimized to study the fundamental properties of the neutrino, at a depth of 2,450 meters off the coast of Toulon (France).

Cables several hundred meters long and equipped with photomultipliers capable of amplifying very small quantities of light are anchored to the seabed at regular distances.

"The interest of water is that when the neutrino interacts in general in matter, it produces electrically charged particles. And if these particles go fast enough in a medium, they can cause the emission of light," explains Sonia El Hedri. This is called the Cherenkov effect.

"Water, because of its transparency, is a particularly favoured medium for detecting this effect," continues the astrophysicist.

On February 13, 2023, a muon, a heavy electron produced by a neutrino, "passed through the entire ARCA detector, inducing signals in more than a third of the active sensors," says KM3NeT, a collaboration bringing together 350 scientists from 21 countries.

The neutrino at its origin had an energy of 220 petaelectronvolts (PeV), or 200 million billion electronvolts. A colossal figure, never observed on Earth.

"That's about the energy of a ping-pong ball falling from a height of one meter," but contained "in a single elementary particle," explained Aart Heijboer, professor at the Netherlands Institute for Subatomic Physics (Nikhef) and member of KM3NeT during a press conference.

Except that a ping-pong ball is made up of trillions of molecules, while this is a single elementary particle that carries the same amount of energy.

Producing such a particle would require an accelerator "all around the Earth at the distance of geostationary satellites," added Paschal Coyle, CNRS research director at the Marseille Particle Physics Center.

"Black hole lurking in the heart of a galaxy? Gamma-ray burst? Supernova?", its origin is being questioned by the CNRS this Wednesday.

With such a level of energy, the origin of the neutrino can only be cosmic. The distance of the event that produced it "is unknown", but "what we are quite sure of is that it does not come from our galaxy", stressed Damien Dornic, researcher at the CPPM.

Astrophysicists have identified twelve blazars , extreme sources of radiation that relentlessly accelerate particles powered by massive, potentially compatible black holes.

It could also be the first detection of a "cosmogenic" neutrino, resulting from "an interaction of ultra-energetic cosmic rays with photons from the intergalactic cosmic background," explained Rosa Coniglione.

Which could help to understand "the composition of these cosmic rays" and "the evolution of the Universe".

"At the time this event happened, our neutrino warning system was still under development," noted Aart Heijboer. By the end of the year, when a new detection occurs, an alert will be sent within seconds "to all telescopes around the world so that they can point in that direction" of the sky and look for a source.