Cosmic Lottery: Despite being electrically neutral and possessing the smallest rest mass among elementary particles (excluding massless photons), neutrinos are among the most abundant particles in the universe. Detecting neutrino interactions with traditional matter is extremely difficult, and identifying an ultra-high-energy neutrino is akin to winning a lottery.
An underwater observatory known as Astroparticle Research with Cosmics in the Abyss (ARCA) has recorded unprecedented subatomic interactions, possibly resulting from an ultra high-energy neutrino descending from the sky. Neutrino physicist João Coelho announced the potential discovery at the Neutrino 2024 conference held in Milan, Italy, stating he would detail the findings in a future paper.
ARCA consists of several plexiglass spheres, each about half a meter wide, attached to strings secured to the Mediterranean seabed southeast of the Italian island of Sicily. The observatory, still being “assembled” 3,500 meters below sea level, is part of the KM3NeT large array of telescopes and detectors, designed as the next generation of neutrino telescopes.
ARCA’s spheres are designed to detect light radiation that can reach the seabed, including high-energy cosmic rays and massless particles like neutrinos. Neutrinos cannot be observed directly, but their presence can be inferred when a neutrino hits an atom of water, air, or rock. When this interaction occurs, the resulting cascade of subatomic particles is detected by ARCA’s instruments.
Ultra-energetic neutrinos have been known to exist for a few years now and are believed to result from some of the most violent events in the universe, such as the growth of supermassive black holes or supernova remnants. These neutrinos can carry half a petaelectronvolt (PeV) or more of energy, while the potential high-energy neutrino announced by Coelho could have an energy level of many tens of petaelectronvolts.
If confirmed, University of Wisconsin-Madison physicist Francis Halzen said the discovery would be a “fantastic event” for particle physics researchers. Coelho described ARCA’s detection – which has been collecting data since the mid-2010s despite still being incomplete – as a phenomenon that’s “very far away” from anything ever recorded on neutrinos.
Neutrinos are some of the most elusive particles known to exist in the universe and the least-studied particles in the Standard Model, and multiple efforts are underway to uncover their secrets. CERN has been using the Large Hadron Collider for neutrino-related experiments, while more observatories designed to detect the highest-energy neutrinos are under construction or have been proposed by researchers around the world.
