The ‘fantastic’ particle could be the most energetic neutrino ever discovered

An observatory still under construction at the bottom of the Mediterranean Sea has detected what may be the most energetic neutrino ever detected. Such ultra-high-energy neutrinos — tiny subatomic particles that travel at the speed of light — have only been known to exist for a decade or so, and are thought to be harbingers of some of the Universe’s most cataclysmic events, such as growing black holes. supermassive in distant galaxies.

Neutrino physicist João Coelho stunned researchers at the Neutrino 2024 conference in Milan, Italy, on June 18 when he revealed the discovery just at the end of his talk.

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The discovery of the neutrino was “a fantastic event,” says Francis Halzen, a physicist at the University of Wisconsin-Madison. He added that the observation highlights the potential of the Astroparticle Research with Cosmics in the Abyss (ARCA) Observatory – a forest of detectors in ‘arrays’ attached to the 3,500m deep sea floor southeast of the Italian island of Sicily.

The neutrino “really stands out, far from everything else,” said Coelho, who is at the AstroParticle and Cosmology Laboratory in Paris. It did not reveal the exact direction the particle had come from, even when the observation occurred: doing so could have tipped off competitors about the neutrino’s possible origins, the researchers said at the conference. Nature. Instead, Coelho promised that these details would be revealed in a letter below. “It would be really interesting to see where in the sky the neutrino originated,” says Nepomuk Otte, a physicist at the Georgia Institute of Technology in Atlanta.

Beads in strings

ARCA is the largest component of a neutrino observatory called the Cubic Kilometer Neutrino Telescope (KM3NeT), which also includes an array from Toulon, France. The cooperation includes European countries along with Morocco, South Africa, Australia, Georgia, China and the United Arab Emirates.

ARCA has been collecting data since the mid-2010s and currently consists of 28 arrays, which the team hopes to expand to 230 by 2028. Each array is 800 meters long and lined with 18 detector units – Plexiglas spheres about half a meter wide, containing light detectors that can each sense only a handful of photons.

Most of the light detected by ARCA is the result of highly energetic cosmic ray particles, which produce showers of electrically charged subatomic particles when they strike Earth’s atmosphere. These particle showers can travel through the water for kilometers and leave behind faint flashes of light, which ARCA is designed to detect.

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The observatory can also detect light from other types of particles, including neutrinos. It does not ‘see’ neutrinos directly. Instead, when a neutrino hits a molecule—of air, water, or underlying rock—it can create a highly energetic charged particle called a muon, which produces a shower of other charged particles as it moves through the detector. Neutrinos can travel through Earth, so the showers of particles they produce can come from any direction, while those from cosmic rays tend to come from the atmosphere. So when ARCA detects a shower from above, it can be difficult to determine the source, but showers that are horizontal or moving upward are more likely to be neutrinos, says Elisa Resconi, a neutrino physicist at the Technical University of Munich in Germany. . .

But for higher-energy neutrinos – those carrying half a petaelectronvolt (0.5×10¹⁵ eV) or more—the Earth acts as a barrier, Resconi says. This leaves a strip of sky around the horizon where Earth particles can be easily detected and distinguished by cosmic rays. “We have this narrow region in which we can see very clean signatures of these neutrinos,” says Resconi, who is part of the collaboration that discovered ultrahigh-energy neutrinos about a decade ago; that group used the much larger IceCube Neutrino Observatory, an ARCA-like detector that is embedded in Antarctic ice.

In his talk, Coelho said more than a third of ARCA’s sensors had recorded flashes consistent with a muon crossing the observatory horizontally, produced by a neutrino arriving from about a degree below the horizon. The particle probably had an energy of many tens of petaelectronvolts, he added – which would make it the most energetic ever detected.

The search continues

At least four other observatories that can detect higher-energy neutrinos are under construction or proposed, according to Naoko Kurahashi Neilson, a neutrino researcher at Drexel University in Philadelphia, Pennsylvania, who described some of the plans in a separate talk . Resconi says she and her collaborators have conducted successful tests for a future neutrino observatory on Vancouver Island, Canada. And Otte is leading another proposed project — with a prototype now being tested in Utah — that would search for Earth-penetrating neutrinos by monitoring the atmosphere just above the horizon for flashes of light.

Until recently, only IceCube had the ability to see these extreme particles—which makes the discovery by ARCA that much smaller all the more sensational. “It’s like winning the big lottery,” says Halzen, who is IceCube’s principal investigator.