Deep beneath the surface of the Mediterranean Sea, Europe”s KM3NeT neutrino telescope has made a groundbreaking discovery by detecting the highest-energy neutrino ever recorded. This monumental event occurred in the early hours of February 13, 2023, when a powerful flash of energy indicated a neutrino with an energy level 30 times greater than any previously observed.
The KM3NeT telescope functions as an elaborate array of sensors that extends one kilometer down to the ocean floor, forming a vast three-dimensional grid. Its primary goal is to capture elusive subatomic particles known as neutrinos, which can traverse the universe without hindrance, even passing through celestial bodies and carrying vital information about distant cosmic phenomena.
Neutrinos, theorized in the 1930s and detected decades later, rank among the most plentiful yet elusive particles in the universe. Billions of neutrinos pass through human bodies each second, undetected. Due to their lack of electric charge and extremely low mass, they interact very minimally with matter, making them exceptionally challenging to observe. This unique characteristic renders them particularly intriguing to physicists. “Neutrinos are the most interesting particles around at the moment,” stated Paschal Coyle from the French National Centre for Scientific Research, who oversees the EU-funded KM3NeT-INFRADEV2 project. “There are lots of mysteries surrounding them. They”re the least understood of the fundamental particles.”
Neutrinos” ability to travel across the cosmos without being absorbed allows them to convey pristine information from some of the universe”s most extreme environments, such as supernovae, black holes, and other cataclysmic events. The study of these particles could unveil secrets regarding the universe”s functioning and the fundamental nature of matter itself. “Neutrinos are the closest thing to nothing we can imagine, but they are key to fully understanding the workings of the universe,” Coyle added.
Whenever a neutrino collides with an atomic nucleus, it generates a cascade of secondary particles. In dense materials like ice or water, this interaction produces a faint blue light known as Cherenkov radiation, which is what KM3NeT“s sensors are designed to detect. This method is also employed by other neutrino observatories, including IceCube in Antarctica and Super-Kamiokande in Japan. While IceCube investigates deep polar ice, KM3NeT observes the dark waters of the Mediterranean.
KM3NeT is a major research infrastructure in Europe, supported by a broad international consortium with funding from the EU and various national sources. It consists of two distinct installations: ARCA (Astroparticle Research with Cosmics in the Abyss), located off the coast of Sicily, targets high-energy neutrinos from outer space, while ORCA (Oscillation Research with Cosmics in the Abyss), near Toulon, France, studies neutrino behavior and mass. Each installation comprises vertical lines of glass spheres the size of basketballs, which house highly sensitive optical sensors.
Currently, more than 1,000 modules have been deployed, with plans to expand to 6,000 by 2027. “It seemed like a crazy idea to build a detector at the bottom of the sea to catch these very weird particles,” remarked Aart Heijboer, a senior physicist at the Dutch National Institute for Subatomic Physics, involved in the telescope”s design. “That caught my imagination.”
The neutrino identified in 2023, designated KM3-230213A, exhibited an astonishing energy charge of 220 petaelectronvolts (PeV) — an unprecedented value for a single particle in the realm of particle physics. “We weren”t really expecting to find such an event,” Coyle noted. “We had to redo a whole load of simulations.” The origins of this remarkable neutrino remain uncertain. Neutrinos can be produced from various sources, including the nuclear reactions that fuel the Sun and the cataclysmic explosions of supernovae.
One hypothesis suggests that the most energetic neutrinos emanate from blazars, which are active galaxies with supermassive black holes that emit jets of energy pointed directly at Earth. Another theory posits that high-energy cosmic rays colliding with photons can also generate neutrinos. If KM3-230213A is indeed cosmogenic, it could indicate that such neutrinos are more prevalent than previously thought. “Or we were just lucky,” Coyle admitted. “It could be that KM3NeT managed to spot a rare, very high-energy neutrino by chance.” Researchers are currently refining their calculations to determine the neutrino”s precise origin.
In the ensuing months, scientists expect to obtain more accurate measurements of its direction. “If it”s coming from a blazar, that”s very exciting. If it”s cosmogenic, that”s also exciting,” Heijboer explained. While ARCA concentrates on identifying the sources of the universe”s most powerful particles, ORCA investigates how neutrinos oscillate among their three distinct “flavors”: electron, muon, and tau. Understanding these oscillations may shed light on the ordering of neutrino masses, a crucial aspect of the Standard Model of physics.
Grasping the nature of neutrinos is essential, as it may help explain why the universe contains matter rather than simply empty space. Following the Big Bang approximately 13.7 billion years ago, matter and antimatter should have annihilated each other, leaving only a void. However, some matter persisted. Neutrinos might provide insights into this enigma, especially if they are demonstrated to be their own antiparticles, a possibility that scientists are keen to explore. “All the experiments that try to measure the difference between a neutrino and an anti-neutrino get confused because they don”t know what the mass ordering is,” Coyle elaborated. “It”s an important input to figuring out why there”s more matter than antimatter.”
By establishing KM3NeT, Europe has positioned itself at the forefront of this global scientific exploration. “Very importantly, we got funding from the EU to do a design study in 2006,” Coyle remarked. This initial support, followed by additional European and national funding, has been pivotal in bringing the project to fruition. The recent detection of KM3-230213A is just the beginning, with more discoveries anticipated as the telescope continues to expand. “We don”t know their mass, we don”t know their mass ordering, we don”t know if they are their own antiparticle,” Coyle concluded. “So neutrinos are where it”s at at the moment.” With thousands of sensors yet to be deployed, KM3NeT is not only enhancing Europe”s role in fundamental research but also listening for some of the faintest signals in nature, each flash potentially holding vital messages about the universe”s origins and the fundamental question of existence.
