Km3net is a neutrino telescope consisting of sting of optical detectors positioned in depth under the ocean. Credit: Km3net
On February 13, 2023, something extraordinary happened in depth under the Mediterranean Sea. The Astroparticle Research of Km3net with the Cosmics telescope in the Abyss (Arca), a wide underwater range of ultra sensitive photoculators, has seen the revealing sign of an incredibly rare cosmic messenger: a highly energetic fundamental particle known as neutrino. With an estimated energy of 220 volts peta-electron (PEV), it is the most energetic neutrin ever detected. The discovery was published on February 12 in Nature.
Scientists did not immediately understand its meaning. It was just another data point in one sea of millions. But as they sift out for months of observations later, something was distinguished on this.
“I understood for the first time how spectacular it was when I looked at our display display,” says Paschal Coyle, researcher of the French National Center for Scientific Research (CNRS) and spokesperson for Km3net at the time of detection. “He had so many more photons (particles of light) of anything we had ever seen.” In fact, Coyle’s program crashed when she tried to analyze the unusually high number of photons. “But it’s more because my programming is not very good,” he says, giving up.
Capture ghost particles
Neutrinos – sometimes called ghost particles – are notoriously elusive. They have no electric charge, barely any mass and can pass through whole planets without interacting. Km3net (abbreviation of the cubic neutrino cubic telescope) is a detector that covers a cubic kilometer (0.24 cubic miles) anchored to the sea bottom 2.5 miles (3.5 km) under the surface, off the southern coast of Sicily.
The setting is brilliant. The strings of glass spheres, each rich in highly sensitive photoculators, await the weaker lightning of blue light, called Cherenkov radiation. This blue glow is emitted when a high energy particle, created by a neutrino that interacts with a particle in the ocean, shut up through the water faster than the speed of light in the water (which is slower than the family member 186,000 miles per second [300,000 km/s] speed of light in the void).
The 220 Pev neutrine probably hit from 6 to 19 miles (from 10 to 30 km) somewhere, creating a slightly lower energy particle called Muone, which then sided through the detector, illuminating over a third of the sensors. The flash lasted about two microseconds, but km3net captured every photon with precision of nanosecondi.
“This was a mega Super-Duper event,” says Coyle. The energy of the neutrinian was out of the graphs and its path was almost horizontal. “Both this combined information is what makes it so special.”
The direction in which this neutrine came makes it particularly intriguing. As mentioned above, neutrinos generally pass straight through everything, including earth. Neutrini telescopes mainly use the planet as a filter to separate cosmic neutrinos from noisy basic events.
But high energy neutrinos are more likely to interact with matter within the earth. They are not likely to travel unharmed through the planet. This means that the only way to detect one is to capture it on one side: to skim through the atmosphere or the surface layers of the earth’s crust.
This is what happened here. The neutrinian arrived almost horizontally, grazing the Maltese continental platform before reaching the detector. “It was exactly the right type of configuration,” says Coyle.
The hunt for cosmogenic neutrinos
For decades, astrophysics have sought the so -called cosmogenic neutrinos. These are super-energy neutrinos created when a cosmic radius created by a high energy event (such as the material that falls into a supermassive black hole or a star that explodes like Supernova) interacts with a low-energy photon of the cosmic microwave background left by the large bang. The analysis of such a neutrino could help scientists better understand many aspects of our universe, but they have not yet been able to detect one.
“This neutrino is certainly in the energy range in which we expect that they are cosmogenic neutrinos,” says Coyle. If it is cosmogenic, it will not indicate a particular object in the sky. This is because the cosmic radius that created it, probably made up of a single proton, would have had its direction bent by the magnetic fields before creating the neutrino.
The neutrinian could also come from a specific astrophysical source, like an active galactic nucleus (Agn), a galaxy with a black supermassion hole in the center that shoots particles jets. Three Agnns close to the position, the astronomers think that the neutrino came on the sky were launching at the time of the detector, says Coyle. Therefore, if the neutrino indicates one of these Agn, it would suggest an astrophysical origin, but if it does not indicate anything, it could be the first cosmogenic neutrinian never detected.
At the moment, the origins of the neutrino are still uncertain. The Km3net team is perfecting their measurements, restricting the region of origin in the sky.
Just to start
The km3net telescope is still under construction. It was just a complete tenth at the time of detection. Currently, the Arca detector consists of only 21 strings of optical sensors. In the end, he will have more than 200. “This is the grandiose thing about neutrinian telescopes,” says Coyle. “You don’t need the complete detector to start collecting interesting data.”
The team is anxious to detect more high energy neutrinos to determine whether it was one -off or the beginning of a new era in astronomy of neutrinos. Other events will help to clarify whether these particles come from known astrophysical sources or if they are truly cosmogenic.
Beyond the hunt for high energy neutrinos, Km3net also manages the oscillation research with Cosmics in the Abyss (ORCA), a second underwater detector under construction near Toulon, France. Orca is optimized for the study of low energy neutrinos to study the fundamental properties of these particles, which we still understand.
Km3net is also an unexpected ally for marine scientists. “Since our detectors sit at the bottom of the sea, we continually monitor the ocean conditions such as the temperature and the oxygen levels and even listen to marine life as whales and dolphins,” adds Coyle. “We offer a free electric plug and a three and a half kilometer ethernet connection in the sea so that other sciences can better study the ocean.”
Neutrinos offer a unique way to explore the universe. Unlike light or charge particles (including the cosmic rays that produce them), they travel in straight lines, unbuttoned by magnetic fields or cosmic dust. If this event aims directly to an AGN, it will help scientists better understand the mysterious jets produced by supermassichi black holes. Otherwise, it could be the first cosmogenic neutrino never detected.
However, this neutrino is a historical discovery. “It’s a completely unexplored range of energy,” says Coyle. “We don’t know what to expect, and this is the most exciting part.”
Km3net is ready to reveal even more on the high energy cosmos. For now, scientists will continue to look at the deep sea, waiting for the next ghost particle to illuminate the darkness.
