Physicists have spotted an extragalactic neutrino about 30 times more energetic than any neutrino detected before, and it may be the first observation of a rare particle born from interactions with the oldest light we can see.
A detector buried off the coast of Malta in the Mediterranean Sea spotted the neutrino—a type of elementary particle that travels at nearly the speed of light and very rarely interacts with other matter. The neutrino’s energy suggests it came from beyond the Milky Way, and physicists believe that extreme astrophysical phenomena or rare interactions between matter and the cosmic microwave background—the universe’s oldest visible light—may be responsible. The team’s research describing the neutrino was published today in Nature.
The detection was made on February 13, 2023—two years ago tomorrow—but it took time for the team to analyze the event and determine the particle’s identity and potential origin.
Neutrinos are one of the most compelling particles in our universe, so much so that governments are pouring billions of dollars into experiments that can detect the little buggers. One such experiment is the Deep Underground Neutrino Experiment (DUNE), a project hosted by Fermilab about one mile beneath the ground in South Dakota.
The Cubic Kilometre Neutrino Telescope, or KM3NeT, made the newly described detection. The telescope comprises two particle detectors situated deep in the Mediterranean Sea—at about 11,318 feet (3,450 meters) and 8,038 feet (2,450 m) beneath the surface, respectively. There, optical modules fastened to the seafloor detect extremely faint light produced when neutrinos interact and create charged particles.
Neutrino detectors must be large (note the “Cubic Kilometre” in the telescope’s name, equivalent to about 0.24 cubic miles) and undisturbed, which is why they’re placed in remote locations like deep underground, at the bottom of the sea, or embedded in ice sheets.
The team registered a muon crossing the detector in February 2023, in a flashy event that triggered signals in more than one third of the detector’s sensors. Based on the trajectory of the particle and its energy, the team believes the muon was derived from a cosmic—as opposed to an atmospheric—neutrino interacting near the detector.
By the team’s calculations, the muon had an energy of about 120 petaelectronvolts (PeV). A single PeV is 1 quadrillion electron volts. As energetic as that sounds, the neutrino that generated the muon is posited to have an even higher energy: 220 PeV. The energy is equivalent to that of a ping pong ball being dropped from 3.4 feet (1 m) above the ground—but shoved into a mere point of matter. And at 220 PeV, the neutrino’s energy is about 30,000 times more powerful than the nominal energy of protons in CERN’s Large Hadron Collider, the most powerful particle accelerator on Earth.
“All this energy is contained in a single elementary point-like particle,” said Paschal Coyle, spokesperson for the KM3NeT experiment at the time of the detection and a researcher at France’s Centre National de la Recherche Scientifique (CNRS) — Centre de Physique des Particules in Marseille, at a Springer Nature press conference held earlier this week. “That’s impressive to us.”
An artist’s impression of the KM3NeT detector, highlighting one of the optical modules. Illustration: Copyright Edward Berbee/Nikhef, Courtesy KM3NeTTo build an LHC capable of producing this particle, you’d need one that wraps around the Earth at the altitude of geostationary satellites, as Coyle explained during the presser.
Neutrinos are vexingly tough to spot. According to the IceCube Neutrino Observatory, about 100 trillion of the particles pass through your body every second. They are the second-most abundant particle in the universe after photons, but despite being incredibly abundant, they’re called “ghost particles” because they almost never interact with matter, making them extremely difficult to detect. Last year, IceCube data revealed seven candidate signals for a particular flavor of neutrino—extracted from nearly 10 years of observatory data, indicating the difficulty researchers have in spotting them. The recent team’s result was induced by just one neutrino, though the observation was made when the detector was just 10% complete, so the team is optimistic that more detections will be made that can yield more information.
The neutrino certainly came from beyond our galaxy, the team found, but beyond that its exact progenitor is unclear. Its most likely source is either cosmogenic—as in, generated through interactions between cosmic rays and photons from the cosmic microwave background, or astrophysical—as in, produced in a stream of particles issuing from one of the universe’s most energetic objects. Specifically, the team nominated 12 blazars, active cores of galaxies that blast jets of subatomic particles at nearly the speed of light, that line up with the approximate direction from where the neutrino appears to have originated.
“It’s incredible—there are these objects in the universe that can accelerate particles to such extreme energies, and how it’s done we don’t fully understand yet,” Coyle added.
Study co-author Damien Dornic, also at CNRS, told Gizmodo in the press conference that the team is reviewing archival data and has requested new observations to determine if characteristics of astrophysical sources indicate the neutrino may have come from one of them, as opposed to a cosmogenic origin.
“In the future, we’ll likely shrink the error box significantly, even for this event,” Aart Heijboer, a physicist at the Nikhef National Institute for Subatomic Physics in the Netherlands and co-author of the research, explained to Gizmodo during the press conference. “If there’s one of these sources directly in the then-much-smaller error box, that’s interesting.”
The observation was made with just 10% of the final detector, Coyle noted, and more events could shed light on the energy, spectrum, and origin of cosmogenic neutrinos and those emanating from active galactic nuclei.
If the neutrino were confirmed to have a cosmogenic origin, the team’s detection is the first of its kind. KM3NeT is currently being expanded, and new results will hopefully either find new events or clarify the nature of the remarkable 2023 observation.
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