The international collaboration that operates the KM3NeT experiment, a powerful telescope submerged in the depths of the Mediterranean, today publishes in Nature magazine the detection of the highest energy neutrino ever captured by a similar experiment. The finding, on the cover of the prestigious magazine, provides the first evidence that such high-energy neutrinos are produced in the universe, although their origin is still unknown. Scientists from the University of Granada are participating in KM3NeT.
On February 13, 2023, the ARCA detector of the KM3NeT underwater neutrino telescope detected an extraordinary event associated with a neutrino of an estimated energy of about 220 PeV (220,000 trillion electron volts, much higher than the particles produced by CERN's LHC). This event, called KM3-230213A, is the most energetic neutrino ever observed to date, and provides the first evidence that neutrinos of such high energies are produced in the Universe. After a long and meticulous process of analyzing and interpreting the data, the KM3NeT collaboration reports today on the details of this finding in an article published in Nature.
The detected event was identified as a muon (an elementary particle related to the electron) that passed through the entire detector, producing a signal in more than a third of the sensors. The inclination of its trajectory together with its enormous energy provides convincing evidence that the muon originated from a cosmic neutrino that interacted in the vicinity of the detector.
"KM3NeT has begun to explore an energy and sensitivity range where the detected neutrinos can be produced in extreme astrophysical phenomena. This first detection of a neutrino of hundreds of PeV opens a new chapter in neutrino astronomy and a new window for observing the universe," said Paschal Coyle, KM3NeT spokesperson at the time of the detection and researcher at the IN2P3/CNRS Particle Physics Center in Marseille (France).
Neutrinos, the most mysterious elementary particles
The high-energy universe is the realm of colossal events such as supermassive black holes, supernova explosions and gamma-ray bursts, events that are still not fully understood. These powerful cosmic accelerators generate streams of particles called cosmic rays, which can interact with the surrounding matter producing neutrinos and photons. During their journey through the universe, the most energetic cosmic rays can interact with the photons of the microwave background radiation, the first light after the origin of the cosmos, to produce extremely energetic neutrinos, called cosmogenic.
"Neutrinos are one of the most mysterious elementary particles. They have no electric charge, almost no mass and interact weakly with matter. They are special cosmic messengers, providing us with unique information about the mechanisms involved in the most energetic phenomena and allowing us to explore the farthest reaches of the universe," explains Rosa Coniglione, deputy spokesperson for KM3NeT at the time of detection and researcher at the National Institute of Nuclear Physics (INFN) in Italy.
Although they are the second most abundant particles in the universe after the photons that make up light, their extremely weak interaction with matter makes them very difficult to detect, and requires huge detectors. The KM3NeT neutrino telescope, currently under construction, is a gigantic infrastructure on the seabed consisting of two detectors, ARCA and ORCA. KM3NeT uses seawater as the interaction medium to detect neutrinos. Its high-tech optical modules detect Cherenkov light, a bluish glow generated by the propagation in water of ultra-relativistic particles resulting from interactions with neutrinos.
This ultra-high-energy neutrino could originate directly from a powerful cosmic accelerator. Alternatively, it could be the first detection of a cosmogenic neutrino. However, based on this single neutrino, it is difficult to draw conclusions about its origin, say the collaboration's scientists. Future observations will focus on detecting more events of this type to build a clearer picture. The ongoing expansion of KM3NeT with additional detection units and the acquisition of new data will improve its sensitivity and increase its ability to identify sources of cosmic neutrinos, making KM3NeT a major player in multi-messenger astronomy.
The University of Granada in KM3NeT
The KM3NeT collaboration brings together more than 360 scientists, engineers, technicians and students from 68 institutions in 22 countries around the world. On behalf of the University of Granada, researchers from the departments of Theoretical and Cosmos Physics and Computer Engineering, Automation and Robotics have been participating in the KM3NeT Collaboration for a decade. Since then, their research has been funded through various programs of the Ministry of Science, Innovation and Universities, as well as regional programs funded by the Regional Government of Andalusia, and through Next Generation EU funds. The UGR works in coordination with researchers from the Institute of Corpuscular Physics (IFIC) in Valencia, the Polytechnic University of Valencia (UPV), the IGIC of the UPV and the Joint Unit of the Spanish Institute of Oceanography (IEO) in KM3NeT.
"The group from the University of Granada that is part of KM3NeT contributes to the experiment in two main aspects. On the one hand, based on the analysis of the data collected by the detector, we work on various physics analyses focused on the search for neutrino sources in the Universe, the detection of dark matter or the study of new physics effects through the measurement of neutrino properties," explains Sergio Navas, one of the principal investigators of KM3NeT at the University of Granada.
"On the other hand, we are participating in the construction of telescope elements focused on the optimal measurement of the time of arrival of signals at optical sensors, which is a key aspect in reconstructing the direction of arrival of neutrinos. We have an infrastructure in the laboratory that allows us to design and apply protocols that ensure that the components we build and install in the experiment meet the required precision requirements (temporal accuracies of less than a billionth of a second)," adds Antonio Díaz García, co-leader of the project at the University of Granada.
"The detection of the KM3-230213A event has been a huge incentive for those of us working on the experiment," says Sergio Navas, "and it is acting as a magnet for new research centers to join the project. At the UGR we continue to work on unraveling the nature of this unique event, about which there are still so many unknowns to be deciphered." The research team is confident that with the full installation of the two KM3NeT detectors, ARCA and ORCA, new light can be shed on the mystery of the origin of cosmic neutrinos.