Chasing neutrinos in Greenland

    -     Nederlands

VUB leads pioneering project to search for cosmic particles from space with radio antennas

Researchers from the Vrije Universiteit Brussel (VUB) are part of a unique installation on the Greenland ice cap that will try to observe almost elusive particles from space. The Radio Neutrino Observatory Greenland (RNO-G) is a pioneering project that uses a new method to detect cosmic neutrinos with very high energy using radio antennas. The first antenna stations have been placed in the ice at the Summit Station research facility.



Nick van Eijndhoven, full professor at VUB and initiator and leader of the project: "We have finally been able to place our first stations in the ice, following all the Covid-19 troubles that made the first installation campaign in 2020 impossible."



The first phase of equipment installation for this ground-breaking project will continue until mid-August. The coronavirus pandemic made this a huge logistical challenge: teams had to spend several weeks in quarantine at different locations before travelling on to Summit Station. The installation of the complete RNO-G observatory - 35 stations in total, 1.25km apart - will take almost four summer seasons. The individual stations can operate autonomously using solar panels and will be connected to each other via a wireless network. VUB researcher Uzair Latif was responsible for aligning and mapping the positions of all the detector elements. He was also part of the team that drilled the first holes in the ice up to a depth of 100m, in which the radio antennas would later be placed. VUB physicist Katharine Mulrey was part of the second team responsible for installing and testing the radio antennas and associated electronics. The third and final team, consisting of researchers from the US and the Deutsches Elektronen-Synchrotron research centre (DESY), will perform the necessary calibrations in the coming weeks, before the Greenland winter sets in and the detector makes its first autonomous observations.



"Neutrinos are extremely elusive, ultralight elementary particles," explains DESY physicist Anna Nelles, co-initiator of the project. "In space, huge amounts of these particles are created, especially during high-energy processes such as those that occur in cosmic particle accelerators. However, neutrinos are very difficult to detect because they hardly ever react with matter. Every second, for example, around 60 billion neutrinos from the sun fly through a fingernail-sized area of the Earth completely undetected."



In extremely rare cases, however, a neutrino does interact with matter when it happens to collide with an atom, such as when passing through the Greenland ice sheet. If the neutrino has enough energy, such rare collisions create an avalanche of secondary particles. Unlike the neutrino, many of these particles are electrically charged. This avalanche of charged secondary particles emits radio waves that can be picked up by the antennas.

The advantage of using radio waves instead of light, as happens at the existing IceCube Neutrino Observatory, is that radio waves can travel much greater distances in the ice, explains van Eijndhoven. "This means we can detect radio signals over distances of several kilometres. This allows us to cover a very large volume of ice in an affordable way and thus gives us a much better chance of detecting such a rare neutrino collision. RNO-G will be the first large-scale radio neutrino detector. Previous smaller-scale experiments in Antarctica have already shown that the technique works and that we should therefore be able to use radio waves to detect these cosmic particles.



"RNO-G and IceCube complement each other perfectly," he adds. "The radio detectors only become sensitive where IceCube’s sensitivity ends due to its limited size, even though that detector covers a volume of 1 cubic kilometre. Furthermore, the sensitivity of both detectors is best for cosmic objects located in the northern part of the sky, albeit with a different energy range. This means that both detectors can study the same sources, where RNO-G can detect the most energetic neutrinos, which are out of range of IceCube, while IceCube can detect the lower energy neutrinos, to which RNO-G is insensitive. This allows us to get a more detailed picture of the astrophysical phenomena involved."



An expansion of the IceCube observatory, IceCube-Gen2, is under way, which will also include radio detectors. "This will open up the southern part of the sky to the most energetic neutrinos," said van Eijndhoven.



More than a dozen partners are involved in the ground-breaking project, including VUB, the University Ghent, the Université Libre de Bruxelles (ULB) and the partnership of VUB and ULB within the Inter-university Institute for High Energies (IIHE), the University of Chicago, Penn State University, the University of Wisconsin-Madison, Ohio State University, Kansas University and DESY. The RNO-G project was made possible by financial support from the FWO-IRI programme.

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