For over a decade, floating cranes have been lowering a strange cargo some 3000 metres under the Mediterranean Sea. The objects look otherworldly: large, shiny spheres crammed with electronics. They are, in fact, detectors for a machine called , designed to search for one of the most mysterious fundamental particles.
The machine is still several years from completion, so got quite a shock when, in 2023, he spotted a dramatic signal in its preliminary data. It was a neutrino, as expected, but one unlike anything ever seen before. “When I first tried looking at this event, my program crashed,” says Coyle, a physicist at the Centre for Particle Physics of Marseille, France.
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KM3NeT had detected a neutrino from space that had about 35 times more energy than any previously seen. It was thousands of times more energetic than anything created in our best particle accelerators. Neutrinos have always defied easy understanding – they interact so faintly with other matter that their presence is normally all but imperceptible. That is behind the decision to place the project’s detectors at the bottom of the sea. But this one seemed almost impossible.
Now the race is on to work out what in the universe could have possibly produced it. As astronomers parse the details, it seems there are two possibilities, both of which point towards some of the deepest and strangest reaches of the cosmos. There is much at stake, as understanding this particle’s origins may help us grasp the true nature of neutrinos and reveal the violent furnaces from which they emanate.
The existence of neutrinos was first predicted by Wolfgang Pauli in 1930 to explain why energy seemed to disappear during radioactive decay. An additional, tiny particle that furtively carried this energy away was deemed to be the culprit. It was later given the name “neutrino”, or “little neutral one” in Italian. But the particle wasn’t experimentally discovered until 1956 because neutrinos have almost no mass and and have no electric charge. This means they rarely interact with matter – a neutrino could pass through a light year’s worth of a dense material like lead without hitting anything.
Even today, neutrinos are especially tricky to study in experiments, says , a neutrino physicist at Harvard University. They only interact via the weak force, one of the four fundamental forces of the universe, along with gravity, electromagnetism and the strong nuclear force. This means the only way to detect them is to observe their effects on the chance occasions when they crash into atomic nuclei. “Neutrinos are one of the most mysterious particles in the periodic table of particle physics, known as the standard model,” says Argüelles-Delgado.
At the outset, the standard model predicted three types, or “flavours”, of neutrino, each of which was massless. Over the past few decades, experiments have revealed that, bizarrely, neutrinos can oscillate between these different flavours as they travel through space, a behaviour that only makes sense if neutrinos have mass. “We are 100 per cent certain they have mass,” says , a neutrino physicist at University College London. In April, the KATRIN experiment in Germany confirmed these masses must be , but their precise values and how neutrinos acquire mass remains unknown.
Particle physicists live in hope that pinning down the peculiar particle’s properties will guide them towards a deeper theory of the fundamental particles and forces. That’s why they are busy constructing gargantuan neutrino-hunting experiments, such as the in the US, in Japan, and in China. Typically, these involve firing artificial beams of high-energy neutrinos into underground detectors to precisely study how neutrinos of different energies oscillate across different distances.
Cosmic neutrinos like the one that crashed Coyle’s computer are another type of beast, one that is of particular interest to researchers because these particles have far greater energies than those created on Earth. They are thought to arise in stellar explosions called supernovae or near the supermassive black holes at the heart of galaxies. But spotting these extreme neutrinos presents an even greater challenge. “When you go to higher energy, you have less neutrinos,” says Argüelles-Delgado. “So, if you want to look for the highest-energy neutrinos, you have to build very large detectors, so large that you cannot build them out of human-made materials.”
Neutrino astronomy
The first such experiment, called , was completed in 2010 at the South Pole. It comprises thousands of basketball-sized detectors that were drilled into a cubic-kilometre of ice. When high-energy neutrinos interact with atoms inside the ice, they produce different particles called muons, which travel faster than the speed of light in ice. This, in turn, creates a kind of sonic boom of light, known as Cherenkov radiation, with a characteristic signature that IceCube’s detectors pick up.
Every day, IceCube spots hundreds of neutrinos. Most of these occur when cosmic rays – other mysterious, high-energy particles that traverse the universe – collide with our atmosphere and produce showers of muons and neutrinos. These atmospheric neutrinos are distinct from cosmic neutrinos, which have themselves travelled across the universe and are much rarer: only a few 100 of them have been detected since IceCube began operation.
That, in itself, has been enough to kick-start a new era of neutrino astronomy, says , a neutrino astronomer at Drexel University, Pennsylvania. “In 2011, we saw neutrinos from outside our solar system for the first time,” she says. Unlike cosmic rays, neutrinos travel in straight lines because of their lack of charge and small mass. That has allowed astronomers to identify some of the cosmic objects that accelerate neutrinos to high energies. In 2018, IceCube traced an incoming neutrino a supermassive black hole at the centre of a galaxy that emits particle jets towards Earth, called a blazar. Other related sources, collectively known as active galactic nuclei, have .
KM3NeT was designed to join in this hunt for cosmic neutrinos, offering a new vantage point from the Mediterranean Sea. Whereas IceCube relies on a large expanse of ice to act as a natural detector, KM3NeT instead opts for water. Upon completion around 2029, it will consist of 345 long chains sunk into the sea, each of which connects 18 spherical light detectors. But in February 2023, when the near-impossible neutrino struck, only a tenth of that had been laid.
“It was really surprising,” says , a researcher at the National Institute for Nuclear Physics, Italy. Many years of data collection at IceCube had seen no evidence of neutrinos whipped up to such energies by “cosmic particle accelerators”. The was about 6 peta-electron volts (PeV), which is about the same energy as a grain of sand falling a few centimetres, but packed into an almost infinitesimally smaller space. So, there was shock when, this February, KM3NeT’s researchers a neutrino that was about 35 times more energetic, at a whopping 220 PeV – although large uncertainties remain over the exact value. “How does a smaller detector that’s been turned on for a shorter period of time see the rarest of them all, the highest-energy neutrino?” says Kurahashi Neilson.
The bolt-from-the-blue nature of this detection has left some researchers worrying that it could be a false positive, an artefact in the detector. Although the result has been checked and rechecked, we can’t yet completely rule this out as a possibility, according to particle physicist at New York University. Assuming it isn’t a false alarm, it requires explanation. “If it’s really 220 PeV, there has to be some additional population [of neutrino sources],” she says.
What could such a source be? One possibility is that the neutrino came from a novel or more extreme kind of cosmic particle accelerator. Perhaps this source, whatever it is, is simply hidden from IceCube’s perspective at the South Pole, but is visible to KM3NeT in the Mediterranean, says Coyle. These additional sources could merely be more extreme versions of the sources that IceCube already detects, says , a particle physicist at the University of California, Irvine. Active galactic nuclei, such as blazars, and supernovae both produce shockwaves that rapidly accelerate protons in a process called Fermi acceleration. High-energy neutrinos are then thought to be produced when these protons crash into other protons or particles of light. So it is plausible that similar processes could whip a neutrino up to as much as 220 PeV. However, no one can say for sure, as the precise mechanisms of neutrino production are often veiled from astrophysicists. “We don’t know how a blazar actually works,” says Li.
To test this idea, the KM3NeT researchers hunted for blazars in the patch of sky where their high-energy neutrino came from and identified 12 possible sources, but none of them was definitive. “They did not identify any convincing source,” says Li.
Another enticing possibility is that KM3NeT’s detection was the first sighting of a background source of the predicted highest-energy neutrinos in the universe, known as cosmogenic neutrinos. Only occurring at energies above 100 PeV, these are thought to be produced during cosmogenic flux, when ultra-high-energy cosmic rays crash into the cosmic microwave background (CMB) radiation – a kind of remnant radiation from the big bang that pervades the universe.
“It is a very exciting possibility,” says Li. “People have predicted this [cosmogenic] flux forever. This must happen, but it has never been detected.” The idea that KM3NeT’s neutrino is cosmogenic also makes sense to Li, who points out it would be against the odds to have seen cosmic neutrinos with a few PeV from active galactic nuclei – as IceCube already has – and then for a similar observation to suddenly leap up to 220PeV. Yet the possibility raises a crucial question: if this cosmogenic flux does exist, why hasn’t IceCube already detected it? No one is quite sure. “It’s challenging to say this event is from a cosmogenic flux” for that reason, says Li.
Cosmic rays
Getting to the bottom of this problem could unlock our understanding of neutrinos and, by extension, the entire cosmos. “There are huge implications for science and astronomy,” says Kurahashi Neilson. Observing cosmogenic neutrinos would allow researchers to better understand the origins of the cosmic rays that produce them, says , a particle astrophysicist at the Niels Bohr Institute in Denmark.
On the other hand, if extreme cosmic accelerators remain the best explanation, high-energy neutrinos may allow us to probe some of the strangest phenomena in the universe. Figuring out what goes on inside supernovae and active galactic nuclei is difficult because the gamma rays they produce are often scrambled by the CMB and by the magnetic fields that permeate space. “What does the violent universe look like? We don’t really know,” says Kurahashi Neilson. However, because neutrinos are unhindered due to their lack of charge and travel in straight lines, we can determine their origin more easily and learn more about how these sources operate.
There might even be galaxies in the universe that don’t emit any visible light or other radiation, perhaps because they are blanketed in dust or obscured by other galaxies, so are only visible by the neutrinos they produce. “A dark star or dark galaxy that you can only see in neutrinos is very possible,” says Kurahashi Neilson. Perhaps there were entire epochs in the early universe, before visible light penetrated the depths of the cosmos, in which neutrinos reigned supreme. “Maybe there was a time in the history of the universe where neutrino emission was [all] you can do,” she says.
Neutrinos might explain where all the antimatter in the early universe went
As our understanding grows of what these cosmic accelerators are, so does the possibility of using them to probe fundamental questions about reality – effectively turning the cosmos into a particle physics laboratory. The energies of these neutrinos and the distances they cover are far greater than anything we could artificially make on Earth, offering a new viewpoint from which to measure their properties. Meanwhile, observing how these neutrinos scatter when they collide with cosmic matter could reveal new cracks in the standard model. “This is a whole new arena in which to look for deviations,” says Li. “It’s very much a hand-in-hand process: if we understand the sources better, that will allow more stringent tests of neutrinos.”
For instance, cosmic neutrino telescopes may one day assist in efforts to uncover the origin of the neutrino’s mass. One possibility is that neutrinos don’t acquire their mass via the Higgs field alone – as all the other particles in the standard model do – but through interactions with exotic, heavy, theoretical neutrinos called Majorana neutrinos, which are their own antiparticles. In this case, the primordial cosmos could have been packed with these Majorana neutrinos, which then decayed asymmetrically, populating the universe with matter instead of antimatter. “If neutrinos are their own antiparticle, that might explain where all the antimatter in the early universe went,” says Nichol.
With so much at stake, Kurahashi Neilson is thrilled at the prospect of having a comparable detector to IceCube in operation. “I’ve been waiting for KM3NeT to come online for a long time,” she says. “This event shows their detector works beautifully. There’s so much more you can do with two detectors versus one. There are simultaneous sources we can measure, and we can collect data faster.”
Kurahashi Neilson is also involved in a separate water-based detector planned for near Vancouver Island in Canada, called the , which would also be comparable to IceCube and KM3NeT. Another in Russia has been under construction since 2021 and could be similarly useful, although collaboration has been difficult since Russia’s invasion of Ukraine. “We only hear from them now in major conferences,” says Argüelles-Delgado. “It’s hard to know what’s going on, which is a pity.”
Regardless, we can surely expect more surprises in the coming years as the extreme universe increasingly comes into focus. “We are moving towards ultra-high-energy neutrino astronomy,” says Tamborra.
Prepare to have your mind blown by CERN, Europe's particle physics centre, where researchers operate the famous Large Hadron Collider, nestled near the charming Swiss lakeside city of Geneva.
CERN and Mont Blanc, dark and frozen matter: Switzerland and France
