The Highest Energy Neutrino Ever Observed

One of the digital optical modules of the KM3NeT neutrino telescope (Picture: Paschal Coyle, CNRS)

The KM3NeT deep-sea neutrino telescope observed an event compatible with an ultra-high-energy neutrino, the highest energy ever measured for this type of event so far.

“The extraordinary discovery of the highest energy neutrino ever observed raises many questions, starting with the mechanism that produced it: we cannot exclude the possibility that we are facing phenomena still unknown,” says Annarita Margiotta, Professor at the ‘Augusto Righi’ Department of Physics and Astronomy at the University of Bologna, coordinator of the Bologna-based KM3NeT research group and Chair Person of the Publication Committee of the international collaboration.

The extraordinary discovery, announced a few days ago in a paper in Nature, provided the first experimental evidence that neutrinos of such high energy are produced in the universe. The KM3NeT/ARCA neutrino telescope, a vast research infrastructure located offshore Portopalo di Capo Passero, Sicily, at a depth of about 3,500 metres, captured this hitherto unique event.

Despite their abundance in the Universe, second only to that of photons, neutrinos are among the most mysterious elementary particles. Their mass is very small, their interaction with matter is weak, and they have no electric charge, characteristics that make them very difficult to detect.

To overcome these obstacles, the KM3NeT experiment uses seawater as an interaction medium: its high-tech optical modules reveal the “Cherenkov radiation,” a glow generated during the propagation of ultrarelativistic particles produced in neutrino interactions.

This is what happened on 13 February 2023: the ARCA detector detected an event (named KM3-230213A) compatible with a neutrino with an estimated energy of about 220 PeV (220 million billion electronvolts). The signal was identified as a single muon that traversed the entire detector, inducing signals in more than a third of its sensors. The inclination of its trajectory, combined with its enormous energy, suggests that the muon originated from a cosmic neutrino that interacted near the detector.

Artistic representation of the event discovered in KM3NeT. The muon, represented by the white line, moves from right to left. The colour indicates the arrival time of the light on the optical sensors, and the size of the “coloured star” is proportional to the signal strength (Image: KM3NeT)

This extraordinary achievement is the result of the KM3NeT Collaboration, bringing together more than 360 scientists, engineers, technicians and students from 68 institutions from 21 countries around the world. Italy's contribution is coordinated by INFN - National Institute of Nuclear Physics, in collaboration with several universities, including a large group of researchers from the ‘Augusto Righi’ Department of Physics and Astronomy at the University of Bologna.

“We started about twenty-five years ago with the ANTARES telescope, offshore the French Riviera, and then continued with KM3NeT, holding positions of responsibility and providing high-level scientific contributions in various fields,” Professor Margiotta says. “Over the years our Bologna group has been enriched with the arrival of young researchers from both universities and INFN, and with several students, PhD students and postdoctoral researchers.”

Today, the KM3NeT infrastructure is being upgraded with additional detection units and the acquisition of new data. These improvements will boost its sensitivity and increase its ability to detect cosmic neutrino sources, making KM3NeT a key instrument for multi-messenger astronomy.

Due to their unique characteristics, neutrinos are cosmic messengers capable of reaching Earth from remote corners of the universe, indicating the direction of their source. Recognising and studying them will provide unique insights into the most energetic astrophysical phenomena, allowing the exploration of the most remote boundaries of the Universe.