High on the Greenland ice cap, researchers are drilling boreholes this week. But these are not Earth scientists looking for clues to past climate. They are particle astrophysicists, looking for cosmic accelerators responsible for the most energetic particles in the universe. By placing hundreds of radio antennas on the surface of the ice and tens of meters below, 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,” said Cosmin Deaconu of the University of Chicago, speaking from Summit station in Greenland.
Detectors elsewhere on Earth occasionally register the arrival of ultra-high energy cosmic rays (HEUs), atomic nuclei that crash into the atmosphere at such high speeds that a single particle can hold as much energy as it does. a well-hit tennis ball. Researchers want to locate their sources, but because nuclei are charged, magnetic fields in space bend their path, obscuring their origins.
This is where neutrinos come in. Theorists believe that as HEU cosmic rays leave their sources, they generate so-called cosmogenic neutrinos when they collide with photons from the diffuse cosmic background, which permeates the universe. Because they are not charged, neutrinos travel to Earth as straight as an arrow. The difficulty comes from catching them. Neutrinos are notoriously reluctant to interact with matter, allowing billions of billions to pass through you every second without warning. Huge volumes of matter must be monitored to capture only a handful of neutrinos colliding with atoms.
The largest of these detectors is the IceCube Neutrino Observatory in Antarctica, which monitors flashes of light from neutrino collisions on 1 cubic kilometer of ice below the South Pole. Since 2010, IceCube has detected many deep space neutrinos, but only a handful – with nicknames such as Bert, Ernie, and Big Bird – that have energies approaching 10 petaelectronvolts (PeV), the expected energy of neutrinos. cosmogenic, explains Olga Botner, an IceCube team member at Uppsala University. “To detect multiple neutrinos with even higher energies in a reasonable time, we need to monitor much larger volumes of ice.”
One way to do this is to take advantage of another signal generated by a neutrino impact: a pulse of radio waves. Since the waves travel up to 1 kilometer in the ice, an array of widely spaced 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), run by the University of Chicago, the Free University of Brussels and the German acceleration center DESY, is the first concerted effort to test the concept. When completed in 2023, it will have 35 stations, each with around 20 antennas, covering a total area of ââ40 square kilometers. The team set up the first station last week near the US-run Summit Station on top of the Greenland Ice Cap, and upgraded to the second. The environment is remote and ruthless. âIf you didn’t bring something, you can’t get it shipped quickly,â says Deaconu. âYou have to make do with what you have. “
The cosmogenic neutrinos the team hopes to capture are believed to emanate from violent cosmic engines. The most likely sources of energy are supermassive black holes, which stuff themselves with matter from surrounding galaxies. IceCube has trace two deep space neutrinos with energies lower than those of Bert, Ernie and Big Bird to galaxies with massive black holes, a sign that they are on the right track. But many more neutrinos at higher energies are needed to confirm the link.
In addition to locating the sources of HEU cosmic rays, the researchers hope the neutrinos will show what these particles are made of. Two major instruments that detect UHE cosmic rays differ in their composition. Data from the Telescope Array in Utah suggests that they are exclusively protons, while the Pierre Auger observatory in Argentina suggests that heavier nuclei are mixed among the protons. The energy spectrum of the neutrinos generated by these particles is expected to differ depending on their composition, which in turn could offer clues as to how and where they are accelerated.
The RNO-G may well capture enough neutrinos to reveal these telltale energy differences, says Anna Nelles of Friedrich Alexander University in Erlangen-NÃ¼rnberg, one of the project leaders, who believes that the RNO- G could capture up to three cosmogenic neutrinos per year. But, “if we’re unlucky,” she said, detections could be so rare that a single detection would take tens of thousands of years.
Even though RNO-G turns out to be a waiting game, it’s also a test bed for a much larger radio network, spread over 500 square kilometers, planned as part of an upgrade to IceCube. If there are cosmogenic neutrinos, the second-generation IceCube will find them and solve the question of what they are. “It could be inundated with neutrinos, 10 per hour,” says Nelles. “But we must be lucky.”