Pioneering project listens to neutrinos from space
GERMAN ELECTRON SYNCHROTRON DESY
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PICTURE: THE FIRST STATION OF THE RADIO NEUTRINO OBSERVATORY ON GREENLAND ICE. THE RED FLAGS INDICATE UNDERGROUND ANTENNAS THAT ARE OPERATED BY SOLAR PANELS (DARK RECTANGULARS). Show more CREDIT: CREDIT: RNO-G, COSMIN DEACONU
In the Greenland ice sheet, a globally unique facility will be listening for extremely elusive particles from space. The Radio Neutrino Observatory Greenland (RNO-G) is a pioneering project that relies on a new method for the detection of very high-energy cosmic neutrinos with the help of radio antennas. The scientists involved in the project have now installed the first antenna stations in the ice of the Summit Station research facility.
“Neutrinos are extremely elusive, ultra-light elementary particles,” explains DESY physicist Anna Nelles, one of the initiators of the project. “These particles are created in huge quantities in space, especially during high-energy processes such as those that take place in cosmic particle accelerators. However, they are very difficult to discover because they hardly react with matter. From the sun alone, around 60 billion neutrinos pass a fingernail-sized spot on earth completely unnoticed – every second. “
The ultra-light elementary particles are sometimes referred to as ghost particles because they can easily penetrate walls, the earth and even entire stars. “This property makes them interesting for astrophysicists because they can be used, for example, to look into exploding stars or merging neutron stars, from which no light can reach us,” explains Nelles, also a professor at the Friedrich-Alexander-Universität Erlangen Nürnberg. “In addition, natural cosmic particle accelerators can be detected with neutrinos.”
In extremely rare cases, however, a neutrino actually interacts with matter if it hits an atom while passing through – for example the Greenland ice sheet. Such rare collisions create an avalanche of secondary particles, many of which, unlike neutrinos, are electrically charged. This cascade of charged secondary particles emits radio waves that can be picked up by the antennas.
“The advantage of using radio waves is that ice is fairly transparent to them,” explains DESY physicist Christoph Welling, who is currently part of the project team in Greenland. “This enables us to detect radio signals over distances of several kilometers.” The greater the range, the greater the volume of ice to be monitored and the greater the chances of discovering one of the rare neutrino collisions. “RNO-G will be the first large-scale radio neutrino detector,” says Welling. Earlier smaller experiments had already shown that it is possible to use radio waves to discover cosmic particles.
The scientists want to install a total of 35 antenna stations at a distance of 1.25 kilometers around the Summit Station on the mighty Greenland ice sheet. Still, it can take months or even years for the observatory to record a signal. “Neutrino research requires patience,” explains Nelles. “Trapping high energy neutrinos is an incredibly rare occurrence. But if you catch one, it reveals an enormous amount of information. ”The researchers are also already anticipating the next step, because the next radioneutrino observatory is planned literally at the other end of the world, which will complement the IceCube neutrino telescope at the South Pole.
An international consortium, to which DESY also belongs, has installed around 5000 sensitive optical detectors several kilometers deep in the Antarctic ice. These photomultipliers keep an eye out for a faint bluish flash of light, which is also generated by the high-energy secondary particles of one of the rare neutrino collisions as they race through the underground ice. With this technology, IceCube has already succeeded in making spectacular observations of neutrinos that arrive, for example, from the vicinity of a huge black hole or a shattered star. The visible light from the subterranean secondary particles cannot be tracked in the ice over such great distances as radio waves. However, the photomultipliers compensate for this by reacting to cosmic neutrinos with lower energies.
“The higher the energy, the rarer the neutrinos, so you need larger detectors,” explains DESY scientist Ilse Plaisier, who is also part of the installation team in Greenland. “The two systems complement each other perfectly: The optical detector grid from IceCube registers neutrinos with energies of up to one quadrillion electron volts, while the array of radio antennas will be sensitive to energies of around ten quadrillion to one hundred trillion electron volts.” The electron volt is in the Particle physics is often used as a unit of energy. One hundred trillion electron volts is roughly the same as the energy of a squash ball moving at 130 kilometers per hour – but in the case of a neutrino that energy is concentrated in a single subatomic particle that is a trillion times lighter than a squash ball.
The first phase of installing the equipment for this pioneering project is scheduled to last until mid-August and was a major logistical challenge during the pandemic: teams had to spend several weeks in quarantine in various locations before reaching the summit station to avoid the coronavirus being introduced . RNO-G will remain on the Greenland ice sheet for at least five years. The individual stations can be operated independently, operated with solar collectors and connected to one another via a wireless network. Based on its operation, it is planned to expand the IceCube neutrino detector at the South Pole as part of its Generation 2 expansion (IceCube Gen2) with radio antennas.
“The detection of radio signals from high-energy neutrinos is a very promising way of significantly enlarging the energy range accessible to us and thus opening this new window to the cosmos even further,” says Christian Stegmann, DESY Director for Astroparticle Physics. “We are going this way via the first test structures in Greenland and will then install radio antennas at the South Pole as part of IceCube-Gen2.”
More than a dozen partners are involved in the pioneering project, including the University of Chicago, Vrije Universiteit Brussel, Penn State University, the University of Wisconsin-Madison and DESY.
DESY is one of the world’s leading particle accelerator centers and researches the structure and function of matter – from the interaction of tiny elementary particles to the behavior of novel nanomaterials and vital biomolecules to the great secrets of the universe. The particle accelerators and detectors that DESY develops and builds at its locations in Hamburg and Zeuthen are unique research tools. They generate the strongest X-rays in the world, accelerate particles to absorb energies and open new windows to the universe. DESY is a member of the Helmholtz Association, Germany’s largest scientific association, and is funded by the Federal Ministry of Education and Research (BMBF) (90 percent) and the federal states of Hamburg and Brandenburg (10 percent).