The most energetic cosmic neutrino ever observed has been detected: what is it and how will it help us understand the Universe?

GenevaResearchers from the European collaboration KM3NeT have detected the most energetic neutrino ever observed. The observation took place on 13 February at the ARCA detector, a powerful neutrino telescope located in the depths of the Mediterranean Sea. The results, published this Wednesday in the journal Nature, are the first evidence that such energetic neutrinos can be produced in the Universe. These particles are very light, have no charge, and interact very little with matter. Although they are the second most abundant particle in the Universe, the characteristics of neutrinos make detecting them very complicated and very advanced technology is required to study them in detail. The discovery represents an important step in understanding the most energetic phenomena in the cosmos.

"KM3NeT has begun to probe the energy range in which neutrinos could be produced by extreme astrophysical phenomena," says Paschal Coyle, spokesperson for the KM3NeT collaboration and researcher at the French National Centre for Scientific Research (CNRS).

The origin of this particle is still to be determined, although researchers are considering two possible explanations. The first is that the neutrino in question was generated by one of the most energetic events in the Universe, such as supernova explosions or supermassive black holes. Another possibility is that it was generated after the collision of a cosmic ray with the cosmic microwave background radiation, the remnant that has filled the entire Universe since it was very young.

A detector on the seabed

Every second, billions of neutrinos from the Sun pass through our bodies, but almost all of them do so without interacting with our bodies. Detecting them requires highly sensitive technology.

The KM3NeT neutrino telescope, which despite the detection is still under construction, is distributed across two detectors, ARCA and ORCA, located underwater near Sicily and off the French Mediterranean coast respectively. Each of them is made up of a hundred spherical detectors anchored to the seabed at a depth of about 2,500 metres, forming a circular array of more than 200 metres in diameter. Placing the neutrino detectors beneath the thickness of the ocean allows them to filter out a large part of the cosmic radiation, which allows for greater sensitivity and greater precision in the observations.

In fact, the particle detected was a muon, a particle similar to an electron but about two hundred times more massive. When passing through water, the muon generates a bluish light called Cherenkov radiation, which hits the detectors. The angle of inclination and the energy with which the muon was observed suggest that it was generated as a result of the interaction of a cosmic neutrino with the seawater near the detector.

The KM3NeT collaboration brings together more than 360 scientists, engineers and technicians from 68 institutions located in 21 countries. “The detection of this event is the result of international collaborative work,” explains Miles Lindsey Clark, KM3NeT project manager at the time of the detection. Collaborators include the Institute for Corpuscular Physics (IFIC) in Valencia, the CSIC and the LAB of the Polytechnic University of Catalonia (UPC).

Carriers of "invisible" information

"Neutrinos are special cosmic messengers. They provide us with essential information about the mechanisms involved in the most energetic phenomena, allowing us to explore the deepest corners of the Universe," explains Rosa Coniglione, spokesperson for the KM3NeT collaboration and researcher at the Italian Nuclear Research Institute.

Events such as black hole collisions, supernova explosions and gamma ray bursts are still largely unknown phenomena. All of these phenomena generate large fluxes of cosmic rays, which travel throughout the Universe producing large quantities of photons and neutrinos. That is why astrophysicists and particle physicists alike are very interested in the detailed study of these particles. Neutrinos, so elusive, could hold the key to answering many of the open questions about the subatomic world and cosmology, such as why there is more matter than antimatter in our Universe.

Understanding the "invisible" universe

Future observations will focus on detecting more such events to build a more accurate map of the Universe. The KM3NeT telescope will thus increase its sensitivity to detect more sources of cosmic neutrinos.

Discoveries such as the one recently made by the KM3NeT collaboration provide essential information for understanding the precise origin of these cosmic neutrinos. The aim of astrophysicists is for neutrinos to become one of the fundamental channels of what is known as multi-message astronomy, which combines information from different sources to provide a deeper understanding of the mechanisms that govern the inner workings of our Universe.