The neutrino observatory KM3Net is located in the depths of the Mediterranean Sea and measures a cosmic neutrino at the highest energies to date
In the Mediterranean Sea, scientists, including astronomers from the Max Planck Institute for Radio Astronomy in Bonn, have discovered a cosmic neutrino with an energy of about 220 PeV using the KM3NeT neutrino telescope. It is the most energetic neutrino ever observed. This discovery proofs that neutrinos with such extreme energies are produced in the universe. Supermassive black holes in the depths of the universe could be a source of extremely fast and extremely high-energy cosmic particles.
An assembly of digital optical modules which later become a part of the KM3Net neutrino telescope in the Mediterranean Sea.
© The KM3NeT Collaboration
On February 13, 2023, an international collaboration, including astronomers from the Max Planck Institute for Radio Astronomy in Bonn, measured a neutrino at record energies in the data from the kilometer-sized neutrino telescope KM3NeT. The energy of the signal is 16,000 times greater than the most powerful particle collisions that have been recreated with the CERN's Large Hadron Collider.
Chasing ghost particles
Visual impression of the ultra-high energy neutrino event observed in KM3NeT/ARCA. The colours indicate the light seen by the "eyes" on each module, where the different colours represent different observing times. The almost horizontally reconstructed track of the particle is shown as a line from left to right.
© The KM3NeT Collaboration
Neutrinos are among the most mysterious elementary particles - they have no electric charge and almost no mass. "They are special cosmic messengers that reveal the secrets of the most energetic phenomena in the universe," adds Rosa Coniglione, deputy spokesperson for KM3NeT at the time of the discovery. Since neutrinos interact only weakly with matter, the KM3NeT telescope uses seawater as a detector volume - soon it will be several cubic kilometers. If a cosmic neutrino reacts with the atomic nuclei of the seawater, muons can be produced. Muons are heavier than electrons but also carry a simple negative charge. The muon acquires so much kinetic energy in this reaction that it produces a cone of light as it plows through the seawater. This Cherenkov light is comparable to the sonic boom that a jet produces when flying at high speed through the atmosphere. The neutrino telescope consists of 230 parallel strings, each with 18 optical modules attached to them like pearls on a necklace. Each module contains 31 photomultipliers that capture and amplify weak light from all directions - including the light generated after a chain of reactions triggered by a cosmic neutrino.
KM3NeT is now detecting neutrinos from extreme astrophysical events, exploring previously uncharted energy ranges. "This first detection of a neutrino in the hundreds of PeV range opens a new chapter in neutrino astronomy," says Paschal Coyle, KM3NeT spokesperson at the time of the detection and a researcher at IN2P3/CNRS in France. One petaelectronvolt (PeV) corresponds to 1015 or one quadrillion electronvolts.
Where did the record-breaking neutrino come from?
The central question is where the high-energy particles that hit the Earth and react in its ocean or atmosphere come from. "By adding observations from other telescopes, we seek to connect the acceleration of cosmic rays, the production of neutrinos, and the role of supermassive black holes in shaping these energetic phenomena," says Yuri Kovalev of the Max Planck Institute for Radio Astronomy. In addition to the environment of supermassive black holes, supernova explosions are also among the candidates for powerful cosmic particle accelerators. The high-energy neutrino that has now been measured could come directly from such an accelerator, or it could be the first detection of a cosmogenic neutrino. Cosmogenic neutrinos could be produced when other cosmic particles react with the weak light of the cosmic microwave background, creating extremely energetic neutrinos. However, since only a single event has been measured here at hundreds of PeV, the origin remains uncertain. To learn more, researchers need to detect more such events.
High-energy particles from space are nothing new
A somewhat smaller neutrino telescope of the same design, Antares, has also measured high-energy neutrinos from space. And there are a number of other creative experiments that have captured the particle bombardment from space. Such as the Pierre Auger Observatory in Argentina, which also measures Cherenkov radiation. In this case, however, the initiators among the cosmic particles are protons that hit the Earth's atmosphere and trigger cascades of secondary particles in it. The muons that are created in the process are not detected in seawater, but in over 1600 water tanks distributed throughout the Argentinean pampas.
BEU
Additional Information
The KM3NeT Collaboration brings together more than 360 scientists, engineers, technicians, and students of 68 institutions from 22 countries all over the world.
KM3NeT has two detectors: ARCA near Sicily and ORCA near Toulon in France. The ARCA detector has 230 units, while the ORCA detector has 115. ARCA units are 700 m high and 100 m apart, while ORCA units are 200 m high with 20 m separations. Each unit contains 18 optical modules with 31 photomultipliers. The data are sent to the shore stations at the INFN Laboratori Nazionali del Sud in Portopalo di Capo Passero and the Laboratoire Sous-marin Provence Méditerranée in La Seyne-sur-Mer via submarine cable.
Following institutions in Germany were involved in the published study: Friedrich-Alexander-Universität Erlangen-Nürnberg with scientists M. Chadolias, Y. Darras, A. Domi, T. Eberl, T. Gal, N. Geißelbrecht, R. Gracia, K. Graf, C. Haack, L. Hennig, O. Kalekin, U.F. Katz, C. Kopper, R. Lahmann, J. Schnabel, J. Schumann, B. Setter, H. Warnofer, and S. Weissbrod; Max-Planck-Institut für Radioastronomie with Y.Y. Kovalev, A. Plavin, and E. Ros; and Julius-Maximilian-Universität Würzburg with S. Buson (also Deutsches Elektronen-Synchrotron DESY), M. Lincetto, and L. Pfeiffer.
MuSES, which stands for Multi-messenger Studies of Energetic Sources, is a pioneering initiative in astrophysics. It is dedicated to the study of Active Galactic Nuclei (AGN), which are among the most powerful particle accelerators known in the cosmos. The MuSES project has received funding from the European Research Council (ERC) under the European Union's Horizon Europe research and innovation programme (grant agreement No 101142396). It is funded by the European Union. Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or ERC. Neither the European Union nor the ERC can be held responsible for them.
