“History shows us that whenever you open a new ‘energy window,’ you never really know what you’re going to find. It’s completely unexplored.”
In a groundbreaking discovery, scientists have detected the highest-energy “ghost particle” ever observed. This particle, a type of neutrino, arrived on Earth traveling at nearly the speed of light and possessed 30 times the energy of the most energetic neutrino detected previously. This discovery marks the first solid evidence that neutrinos with such extraordinarily high energies can be produced somewhere in the universe.
The immense energy of this particle suggests that it originated from outside our Milky Way galaxy. While its source remains uncertain, the research team has narrowed it down to 12 potential candidates. These suspects are all “blazars,” the powerful cores of “active galactic nuclei” (AGN) powered by supermassive black holes that feed on surrounding matter. Blazars are a type of quasar that stands out because they emit beams of high-energy particles and radiation directed straight at Earth.
However, an alternative explanation posits that this high-energy neutrino may have been created when a massive cosmic ray particle collided with photons—light particles—left behind from the universe’s early moments following the Big Bang.
The neutrino was detected through the observation of a single muon, an elementary particle, by the Kilometer Cubic Neutrino Telescope (KM3NeT), located 11,300 feet (3,450 meters) beneath the surface of the Mediterranean Sea. The event, designated KM3-230213A, occurred on February 13, 2023. During this event, the muon traversed the full extent of the KM3NeT detector and activated one-third of its many sensors.
“This neutrino is very likely of cosmic origin, and its energy is such that it’s in a completely unexplored region of energy,” said Paschal Coyle, a researcher from the French National Centre for Scientific Research, during a press conference held on February 11. “Whenever you open a new ‘energy window,’ you never really know what you’re going to find. It’s completely unexplored.”
Neutrinos: The Elusive Ghost Particles
Neutrinos are often referred to as “ghost particles” due to their lack of charge and negligible mass. In fact, approximately 100 trillion neutrinos pass through your body every second without you even noticing. Despite being the second most abundant particle in the universe after photons, neutrinos are notoriously difficult to detect, requiring specialized detectors placed deep underground or underwater, like KM3NeT, which is submerged deep under the Mediterranean Sea.
“Neutrinos are among the most mysterious elementary particles,” said Rosa Coniglione, a KM3NeT team member from the Istituto Nazionale di Fisica Nucleare in Italy. “They have no electric charge, almost no mass, and interact only weakly with matter. 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 KM3NeT detected a flash of light from a muon—an electron-like particle, rather than a neutrino—it was the properties of this elementary particle that revealed it had been created when a high-energy neutrino collided with another particle.
“There are many muons passing through the detector from above, created in the Earth’s atmosphere, but these are not particularly interesting. We detected roughly 110 million of them in 2023,” explained Aart Heijboer, the experiment’s physics coordinator at the time of the detection. “But this particle was oriented horizontally, and to produce a horizontal muon, it must have been a neutrino because these are the only particles that can travel through the required 87 miles (140 kilometers) of rock and water to produce this horizontal particle in the detector.”
The team was able to measure the energy of the neutrino from the amount of light recorded by the detector. The energy of this neutrino was found to be 220 million billion electron volts, which is 30,000 times the energy of the Large Hadron Collider (LHC), Earth’s largest particle accelerator. To put this into perspective, Coyle explained that to accelerate a particle to such high energies, the LHC would need to be expanded from its current 17 miles (27 kilometers) to a staggering 25,000 miles (40,000 kilometers)—the circumference of Earth.
“It would require a global LHC accelerator all around the world to reach such an energy,” Coyle added.
Searching for the Cosmic Particle Accelerator
The question remains: What natural cosmic accelerator could produce a neutrino with such extraordinary energy? Although researchers don’t yet have a definitive answer, they suspect that the answer may lie in the heart of active galactic nuclei (AGN).
The universe’s high-energy events are dominated by incredibly powerful phenomena such as supernova explosions—when massive stars end their lives in violent death throes—and gamma-ray bursts, brief but intense explosions of high-energy light. Despite their brief duration, gamma-ray bursts release more energy than the sun will radiate over its entire lifetime.
While these events could act as natural particle accelerators, the prime suspects for this particular neutrino are supermassive black holes. These behemoths, millions or even billions of times the mass of our sun, are surrounded by vast amounts of matter in AGNs. In turn, AGNs are known as “quasars,” which emit powerful jets of matter that can stretch across hundreds of light-years. When these jets are directed straight at Earth, they are known as “blazars.”
The jets emitted during blazar flare events are composed of high-energy cosmic rays that can extend well beyond the limits of the galaxies housing the supermassive black hole that created them. These jets also consist of electromagnetic radiation, ranging from low-energy radio waves to high-energy gamma rays. When these particles strike others within the originating galaxy, they create showers of high-energy neutrinos that cascade across the cosmos.
Coniglione explained that, by measuring the direction of the particle, the researchers were able to trace it back to the edge of the Milky Way galaxy. However, the search for the exact source of this high-energy ghost particle within our galaxy proved fruitless. The team identified 12 potential sources: all of them blazars situated beyond the boundaries of the Milky Way. One of these 12 may be the origin of the newly discovered particle.
An Alternative Hypothesis: The Cosmogenic Neutrino
While the team suspects a blazar may be the source of this neutrino, another theory is under consideration. The high-energy neutrino may have been created when an ultra-high-energy cosmic ray—most likely a proton—collided with photons in the cosmic microwave background (CMB), a remnant from the universe’s early moments. This ancient light, emitted just after the Big Bang, is the first radiation capable of freely traveling through space.
An interaction between a cosmic ray and the CMB would produce a shower of high-energy neutrinos. If this hypothesis is correct, this would represent the first detection of a “cosmogenic neutrino.” While scientists are certain that such neutrinos exist, they have remained elusive until now.
The potential discovery of a cosmogenic neutrino is particularly exciting because it could open up a new form of astronomy. It could bring together different domains of cosmic research, combining “traditional astronomy,” which relies on electromagnetic radiation, with gravitational wave astronomy—focused on the tiny ripples in spacetime. This combination would represent a new branch of “multi-messenger astronomy” that uses neutrinos to probe high-energy phenomena.
The Future of Neutrino Astronomy
Currently, with only one detection of a high-energy neutrino, the team is not yet able to distinguish whether this neutrino originated from a cosmic particle accelerator like a blazar, or from a cosmic ray-CMB collision. However, the fact that KM3NeT was able to make this groundbreaking discovery while still under construction provides optimism that this cosmic mystery could soon be unraveled.
“In the next year, KM3NeT will deliver more and more data with improved angular resolution,” Coniglione said. “In the near future, we will have a more refined understanding of the origin of this event and likely a firmer conclusion.”
The team’s findings were published on February 12 in the prestigious journal Nature. The work represents a significant step forward in our understanding of the universe’s most energetic phenomena and provides a glimpse into the future of neutrino-based astronomy, a field that promises to reveal more mysteries of the cosmos in the years to come.