Record-breaking Ghost-like Particle Found in the Mediterranean Sea – Sky & Telescope

Astronomers have used a giant, underwater telescope to detect the most energetic neutrino ever observed. Named KM3-230213A, the tiny particle carried 30 times more energy than the previous record holder.

Neutrinos are ghostly subatomic particles that rarely deign to interact with ordinary matter. In fact, a light-year-long bar of lead would only have a 50:50 shot at halting a speeding neutrino. Understandably, detecting them requires scientists to go to extraordinary lengths.

Such Herculean efforts include burying detectors thousands of meters below the surface of the Mediterranean Sea. This dark locale, where even sunlight cannot reach, is home to the Kilometre Cubic Neutrino Telescope (KM3NeT). It is still under construction, but already consists of two sites: Astroparticle Research with Cosmics in the Abyss (ARCA) and Oscillation Research with Cosmics in the Abyss (ORCA). At each site, dozens of detectors called photomultipliers dangle from hundreds of strings anchored to the seabed.

On February 13, 2023, ARCA picked up a single muon that triggered more than a third of the site’s intricate sensors. The inbound trajectory of the muon, coupled with its extremely high energy, led KM3NeT researchers to conclude that the muon was the result of a 220 peta electrovolt (PeV) neutrino striking the Earth’s atmosphere from space. The team’s results are published in Nature.

“This first ever detection of a neutrino of hundreds of PeV opens a new chapter in neutrino astronomy and a new observational window on the Universe,” says Paschal Coyle (National Institute of Nuclear Physics and Particle Physics, France).

Such finds not only tell us how much energy it is possible for neutrinos to have, but they also deliver other key insights. “[Neutrinos] are special cosmic messengers, bringing us unique information on the mechanisms involved in the most energetic phenomena and allowing us to explore the farthest reaches of the Universe,” says Rosa Coniglione (National Institute for Nuclear Physics, Italy).

The exact source of this neutrino remains unclear. It could have come from a so-called cosmic accelerator. Cataclysmic events such as supernovae explosions, gamma-ray bursts, and feeding black holes can whip up subatomic particles to extremely high energies. “We know so little about [these] extreme processes . . . that any extra information we can get is very valuable,” says Patrick Dunne (Imperial College London), who was not involved in the research.

Alternatively, this record-breaking neutrino could be cosmogenic. Astronomers suspect that cosmogenic neutrinos are produced when high-energy cosmic rays interact with the photons of the cosmicmicrowave background, the leftover light from the Big Bang. If confirmed — a sizeable “if” at this early stage — it would be the first cosmogenic neutrino that astronomers have ever seen.

It’s also possible, given the high energy of this neutrino, that it may have come from somewhere else entirely. “It leads us to question whether it comes from a process we know about or something we haven’t seen before,” says Dunne.

Astronomers will need more than a single observed neutrino to decide. There could soon be more as the construction of KM3NeT continues. “The scale of KM3NeT, eventually encompassing a volume of about one cubic kilometre with a total of about 200,000 photomultipliers, along with its extreme location in the abyss of the Mediterranean Sea, demonstrates the extraordinary efforts required to advance neutrino astronomy and particle physics,” says Miles Lindsey Clark (Astroparticle and Cosmology Laboratory, France).

If this early discovery is anything to go by, scientists could be handsomely rewarded for those efforts.