Flags mark the locations of antennas designed to detect radio pulses from neutrino collisions in the ice.
CHRISTOPH WELLING/RNO-G COLLABORATION/DESY
High on Greenland’s ice sheet, researchers are drilling boreholes this week. But they are not earth scientists seeking clues to the past climate. They are particle astrophysicists, searching for the cosmic accelerators responsible for the universe’s most energetic particles. By placing hundreds of radio antennas on the ice surface and dozens of meters below it, they hope to trap elusive particles known as neutrinos at higher energies than ever before. “It’s a discovery machine, looking for the first neutrinos at these energies,” says Cosmin Deaconu of the University of Chicago, speaking from Greenland’s Summit Station.
Detectors elsewhere on Earth occasionally register the arrival of ultra–high-energy (UHE) cosmic rays, atomic nuclei that slam into the atmosphere at speeds so high that a single particle can pack as much energy as a well-hit tennis ball. Researchers want to pinpoint their sources, but because the nuclei are charged, magnetic fields in space bend their paths, obscuring their origins.
That’s where neutrinos come in. Theorists believe that as UHE cosmic rays set out from their sources, they spawn so-called cosmogenic neutrinos as they collide with photons from the cosmic microwave background, which pervades the universe. Because they are not charged, the neutrinos travel to Earth as straight as an arrow. The difficulty comes in catching them. Neutrinos are notoriously reluctant to interact with matter, which allows trillions to pass through you every second without any notice. Huge volumes of material have to be monitored to capture just a handful of neutrinos colliding with atoms.
The largest such detector is the IceCube Neutrino Observatory in Antarctica, which watches for flashes of light from neutrinoatom collisions across 1 cubic kilometer of ice beneath the South Pole. Since 2010, IceCube has detected many deep space neutrinos, but only a handful—with nicknames including Bert, Ernie, and Big Bird—that have energies approaching 10 petaelectronvolts (PeV), the expected energy of cosmogenic neutrinos, says Olga Botner, an IceCube team member at Uppsala University. “To detect several neutrinos with even higher energies within a reasonable time, we need to monitor vastly larger volumes of ice.”
One way to do that is to take advantage of another signal generated by a neutrino impact: a pulse of radio waves. Because the waves travel up to 1 kilometer within ice, a widely spaced array of radio antennas near the surface can monitor a much larger volume of ice, at a lower cost, than IceCube, with its long strings of photon detectors deep in the ice. The Radio Neutrino Observatory Greenland (RNO-G), led by the University of Chicago, the Free University of Brussels, and the German accelerator center DESY, is the first concerted effort to test the concept. When complete in 2023, it will have 35 stations, each comprising two dozen antennas, covering a total area of 40 square kilometers. The team installed the first station last week near the U.S.-run Summit Station, at the apex of the Greenland Ice Sheet, and has moved on to its second. The environment is remote and unforgiving. “If you didn’t bring something you can’t get it shipped quickly,” Deaconu says. “You have to make do with what you have.”
The cosmogenic neutrinos the team hopes to capture are thought to emanate from violent cosmic engines. The most likely power sources are supermassive black holes that gorge on material from their surrounding galaxies. IceCube has traced two deep space neutrinos with energies lower than Bert, Ernie, and Big Bird to galaxies with massive black holes—a sign they are on the right track. But many more neutrinos at higher energies are needed to confirm the link.
In addition to pinpointing the sources of UHE cosmic rays, researchers hope the neutrinos will show what those particles are made of. Two major instruments that detect UHE cosmic rays differ over their composition. Data from the Telescope Array in Utah suggest they are exclusively protons, whereas the Pierre Auger Observatory in Argentina suggests heavier nuclei are mixed among the protons. The energy spectrum of the neutrinos spawned by those particles should differ depending on their composition—which in turn could offer clues to how and where they are accelerated.
RNO-G just might catch enough neutrinos to reveal those telltale energy differences, says Anna Nelles of Friedrich Alexander University Erlangen-Nürnberg, one of the project leaders, who estimates that RNO-G might catch as many as three cosmogenic neutrinos per year. But, “If we’re unlucky,” she says, detections might be so scarce that scoring just one would take tens of thousands of years.
Even if RNO-G proves to be a waiting game, it is also a testbed for a much larger radio array, spread over 500 square kilometers, planned as part of an IceCube upgrade. If cosmogenic neutrinos are out there, the second generation IceCube will find them and resolve the question of what they are. “It could be flooded with neutrinos, 10 per hour,” Nelles says. “But we have to be lucky.”