It's hard to think of anything more elusive and mysterious than neutrinos. These subatomic "ghost particles" carry no electric charge and are almost weightless, allowing them to stream through the universe at nearly the speed of light while scarcely interacting with ordinary matter. That makes them very hard to detect—but also invaluable for revealing the otherwise-inaccessible innards of stars, active galactic nuclei, and other extreme astrophysical systems where they're produced in abundance.
To find such slippery quarry, physicists have built enormous detectors deep underground, buried in Antarctic ice, and even beneath the ocean waves. One such facility, the Kilometer Cube Neutrino Telescope, or KM3NeT, looks for neutrinos via sprawling arrays of instruments tethered to the floor of the Mediterranean Sea off the coast of Sicily. Although only partially constructed, KM3NeT made a historic find in early 2023 when it recorded a neutrino of unprecedented energy bashing into molecules of seawater. Scientists are scratching their heads over how to explain the discovery, which was announced in a press conference earlier this week alongside an associated paper published in Nature.
This single particle, the KM3NeT team's analysis suggests, carried some 120 peta electronvolts of energy—about "the energy of a falling Ping-Pong ball," as our senior reporter Meghan Bartels writes in her story covering the stunning result. "But the energy of a Ping-Pong ball is spread over a thousand billion billion particles. Here, squeezed into one of the tiniest flecks of matter in our universe, that energy amounted to tens of thousands of times more than what can be achieved by the world's premier particle accelerator, the Large Hadron Collider at CERN."
This single ultrahigh-energy neutrino is as much as thirty times more energetic than the previous record-holder. How did it gain so much energy? Where did it come from? Why have no other neutrino detectors found similarly potent particles? There are far more questions than answers at this point, but if or when certainty comes it could spark significant advances in our understanding of these spooky particles and their potentially profound collective influence on the universe at large.
—Lee Billings
