A Detector at the Bottom of the Sea Found an Extraordinary Signal From the Unseen Universe

Neutrinos are arguably the most enigmatic particles in the universe, but scientists on Earth are getting better at detecting them.
In February of 2023, the underwater Cubic Kilometer Neutrino Telescope (KM3NeT) detected a high-energy neutrino with 30 times more energy than any previously detected neutrino.
Amazingly, KM3NeT detected this particle while under construction, using only 20 percent of its photodetectors.
Neutrinos lie at the frontier of scientific unknowns about the universe. However, there's a problem—neutrinos also like to keep to themselves. They're the ultimate recluses of the particle physics world, but scientists have developed tools over decades to detect the reactions they can sometimes set off.
One of those detectors is known as the Cubic Kilometer Neutrino Telescope (KM3NeT), which comprises two detector arrays anchored to the floor of the Mediterranean Sea. The Astroparticle Research with Cosmics in the Abyss (ARCA) array is located off the coast of Sicily, and in the middle of the night on February 13, 2023, the installation recorded an unusual signal—a high-energy muon streaking through the array in just a few microseconds.
From the data, scientists determined that the muon contained 120 peta-electron volts (PeV) of energy, and further extrapolated that the instigating neutrino must have contained energies of 220 peta-electron volts—a level that's 30 times higher than any neutrino ever detected. The results of this astounding discovery were published in the journal Nature.
'Neutrinos are the closest thing to nothing that we can imagine,' Paschal Coyle, Centre national de la recherche scientifique (CNRS) researcher and KM3NeT spokesperson at the time of the detection, said during a press conference earlier this week, 'but they are key to fully understanding the workings of the universe.'
Scientists don't directly detect neutrinos, but they can infer things about them by analyzing their interactions with the weak nuclear force (neutrinos also interact with gravity, but the effects are negligible). To give you a sense of how difficult it can be to detect a neutrino, scientists estimate that if 10 trillion neutrinos generated from the Sun pass through the Earth, only one will interact with a particle and produce a detectable reaction. Put another way, you could construct a lead wall five light years in width, and 50 percent of neutrinos could still pass through unscathed.
So, not only did it come as a surprise when the ARCA array lit up in February of 2023, it was almost unbelievable that the experiment—which, at that time, had only deployed 10 percent of its photoreceptors—detected something as extraordinary as KM3-230213A (the name of the neutrino event).
Two things make this particular muon (and, by extension, the neutrino that created it‚, particularly interesting. The first is its high energy level, which suggests a cosmic origin. The second is the angle of its trajectory. Because it was close to the horizon, it's likely that the neutrino collided with an atom in the deep sea surrounding the detector.
'Neutrinos are one of the most mysterious of elementary particles. They have no electric charge, almost no mass and interact only weakly with matter,' Rosa Coniglione from KM3NeT said in a press statement. 'They 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.'
Although the researchers can't be sure of the neutrino's origin, it's likely that the particle originated from a cataclysmic event like a gamma-ray burst, accreting supermassive black hole, or supernova explosion. It's also possible that energetic cosmic rays that ferried this neutrino to us interacted with protons found in the cosmic microwave background radiation, creating what's known as a 'cosmogenic neutrino.'
KM3NeT is only at the beginning of its journey—this discovery popped up when the device was using only 21 of its planned 230 detection lines. And scientists are hopeful that this high-energy particle will be only the first of many similar discoveries. The project will also get a major assist in the exploration of neutrinos from the Deep Underground Neutrino Experiment, or DUNE, when it goes online in the coming years.
Neutrinos may be the universe's most reclusive particles, but scientists are trying their best to bring some of their anti-social behaviors to light.
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