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Strings of detectors, lowered into Russia’s Lake Baikal, the world’s deepest lake, will form one of several neutrino telescopes.

Bair Shaybonov/Baikal-GVD

Neutrino hunters go underwater in quest to trap ghost particles

Since 2010, IceCube, a detector frozen in the ice beneath the South Pole, has snared neutrinos from deep space. The universe is awash with these fleeting, almost massless particles, but IceCube is after a rare subset. They are messengers from distant cosmic accelerators such as supernovae, neutron stars, and black holes. IceCube has caught about 300 in its cubic kilometer of ice, but has had less success tracing them to their probable source—just two so far. Now, it is poised to get help from new detectors that trade Antarctic ice for deep northern waters.

This month, researchers will begin to drop sensor strings into the Mediterranean Sea off the coast of Sicily, as they embark on building the Cubic Kilometre Neutrino Telescope (KM3NeT). Meanwhile, a Russian team has been working on the frozen surface of Lake Baikal in Siberia, the world’s deepest lake, to drop detector strings into its depths. The Gigaton Volume Detector (Baikal-GVD) is already half complete and taking data. A third effort, the Pacific Ocean Neutrino Explorer (PONE) hopes to deploy one or more prototype strings off the west coast of Canada next year.

“We’re really looking forward to having a worldwide network,” says Olga Botner, an astroparticle physicist at Uppsala University and IceCube team member. “With three detectors we’ll get more neutrinos and more likelihood of identifying sources.”

Trillions of neutrinos stream unnoticed through your body every second, most of them low-energy neutrinos from local sources like the Sun. IceCube and the other “neutrino telescopes” study the rare high-energy neutrinos produced when charged particles—cosmic rays—accelerated to ultrahigh energies in the distant universe smash through a cloud of gas. The cosmic rays can also reach Earth, but can’t easily be traced back to their source because they follow a twisting journey through the universe’s magnetic fields. Chargeless neutrinos offer a truer flight path that reveals their source. But only if researchers can catch them.

Very occasionally a passing neutrino will collide with an atomic nucleus, spawning other particles. In water or ice, those particles emit a flash of light as they slow down. IceCube contains more than 5000 light detectors watching the deep, transparent ice to pin down the timing and brightness of the flash, from which researchers can reconstruct the neutrino’s energy and path.

IceCube catches about 30 high-energy neutrinos per year that are presumed to be extragalactic. That’s about the number expected to come from supernovae in starburst galaxies—young galaxies that forge huge, fast-burning stars tens of times faster than the Milky Way. When these stars die and explode, they are thought to fling out cosmic rays that produce neutrinos when they crash through dense clouds of star-forming gas near the supernovae.

The rate at which IceCube detects extragalactic neutrinos is “a strong hint that these are the sources,” says theorist Eli Waxman of the Weizmann Institute of Science. Yet so far, the two neutrinos to be traced back to likely sources seem to have come from supermassive black holes (SMBHs) in galactic cores, not starbursts. One seemed to come from a blazar, a jet from an SMBH pointing at Earth, and another, announced earlier this year, from a tidal disruption event—an SMBH tearing apart a star. To resolve the issue, Waxman says, researchers need bigger detectors and better pointing. “With this next generation we will identify individual starburst galaxies,” he says.

Traps of water and ice

Neutrino telescopes need huge detection volumes to catch the elusive particles. Two underwater detectors should offer better pointing than IceCube, enabling astronomers to trace the particles back to their origins.

Flashes in the darkRarely, a neutrino will strike a nucleus, spawning particles that slow down and emitlight. The neutrino’s energy and path can be reconstructed from the flash. 2700 m Cubic Kilometre Neutrino Telescope Mediterranean Sea Volume: ~1 km 3 IceCube Antarctica Volume: ~1 km 3 Baikal Gigaton Volume Detector Siberia Volume: ~0.7 km 3 3400 m 1450 m 2450 m Light detector Buoy Anchor 700 m 1240 m Single unitout of 14 Cherenkovlight Light detector Support wire Muon Collision with atomin water molecule Neutrino

Constructing IceCube took 5 years of drilling into the Antarctic ice cap with hot-water jets. Building a detector deep underwater has its own challenges. Each KM3NeT string, studded with detectors 40 meters apart, is dropped from a ship as a ball and unspools as it sinks to the floor of the Mediterranean 3.4 kilometers down. Buoys keep the strings upright, while a remotely operated submersible anchors them and connects them into power and communication networks. The team is preparing to install 18 strings by September. “It’s a major step forward,” says spokesperson Paschal Coyle of the Center for Particle Physics of Marseille. The aim is to have 230 strings and more than 4000 light detectors in place by 2026 to make a detector slightly larger than IceCube.

Baikal-GVD researchers have an easier job. For now, they can safely drive onto the frozen lake, erect winches, and lower strings into the water. Working on the ice “really makes it easier and cheaper to deploy things,” says Dmitry Zaborov of the Russian Academy of Sciences’s Institute for Nuclear Research. The team has installed 56 strings so far and is aiming for another 40 by 2024, to cover a volume about 70% the size of IceCube.

Using water instead of ice will give the new detectors an edge. Light scatters less in water, so particle tracks can be mapped more precisely, giving a sharper view of the neutrinos’ origin. KM3NeT estimates it can achieve a top angular resolution of less than 0.1°, compared with IceCube’s 0.5°, which is about the size of the full Moon.

The telescopes’ location in the Northern Hemisphere is also a plus. Neutrino detectors look down rather than up, watching for neutrinos that have passed through Earth, which acts as a shield against many background particles. As a result, IceCube’s view takes in the northern sky. The northern detectors, in contrast, will look south, into the heart of the Milky Way, the most likely home for neutrino sources such as magnetized neutron stars, the galaxy’s SMBH, or, if astronomers are lucky, a new supernova.

Later in the decade they could be joined by P-ONE, which is taking advantage of a network of existing power and data cables installed for oceanographic experiments off the coast of British Columbia. “It’s a plug-and-play operation,” says team leader Elisa Resconi of the Technical University of Munich. With three widely spaced telescopes in the north, “we’ll see nearly the entire sky all the time,” Resconi says. “It will bring the field to a new level.”

The ultimate aim, once researchers can link neutrinos of particular energies to different types of sources, is to do true neutrino astronomy: viewing the universe not with photons, but with neutrinos, which bear news about violent corners of the universe otherwise hidden from view. As Botner puts it: “We want to see the parts of the universe that cannot be seen with photons.”