Neutrinos and antineutrinos, the antimatter twin of a neutrino, are ghostly subatomic particles that can be found in the most extreme environments in the universe. On 6 December 2016, for example, a high-energy particle raced from space to Earth at a speed approaching the speed of light and carrying 6.3 petaelectronvolts (PeV) of energy. Deep in the ice cap at the South Pole, it collided with an electron and produced a particle that rapidly decayed into a shower of secondary particles. This interaction was captured by the IceCube Neutrino Observatory, a huge telescope buried in Antarctica’s glacier. IceCube had observed a Glashow resonance, a phenomenon predicted in 1960 by Nobel laureate Sheldon Glashow.
Glashow first proposed this resonance in 1960. In a paper, he predicted that an antineutrino could react with an electron to produce a yet undiscovered particle – if the antineutrino had the right energy. When that particle, the W boson, was discovered in 1983, it turned out to be very heavy. The Glashow resonance would require a neutrino with an energy almost 1,000 times greater than that which CERN’s Large Hadron Collider can produce. In fact, no constructed particle accelerator on earth can produce a neutrino with that much energy.
But our universe does have such natural accelerators. Supermassive black holes at the centres of galaxies and other extreme cosmic events can generate particles with energies that are impossible to reach on Earth. Such a phenomenon was likely responsible for the 6.3 PeV antineutrino that reached IceCube in 2016. Glashow, now professor emeritus of physics at Boston University, stresses the need for more detections of Glashow resonance events. “To be absolutely sure, we would need to see another such event at exactly the same energy as the one observed,” he says. “So far there is one, and one day there will be more.”
VUB aims to detect more cosmic neutrinos with innovative radio technology
Since IceCube became fully operational in May 2011, the observatory has detected hundreds of high-energy cosmic neutrinos. But the 2016 antineutrino is only the third neutrino with an energy above 5 PeV detected by IceCube. It is also the first time that the measurements have been able to separate neutrinos from antineutrinos, which has important implications for future measurements. “There are some properties of astrophysical neutrinos that we cannot measure, such as the physical size of the accelerator and the magnetic field strength in the acceleration region,” said Tianlu Yuan, a principal analyst. “If we can determine the ratio of neutrino to antineutrino, we can investigate these properties.”
In the future, the IceCube Collaboration aims to detect even more high-energy particles and make decisive measurements of the neutrino-to-antineutrino ratio. VUB’s IceCube team is taking a lead in this, under the guidance of Nick van Eijndhoven and Krijn de Vries. They are currently developing an innovative detection technique, based on radio signals produced during the process of a neutrino interaction.
The IceCube consortium recently announced an upgrade of the detector to be implemented in the coming years, as the first step to IceCube-Gen2. What this research mainly shows is the value of international cooperation. IceCube is operated by more than 400 scientists, engineers and employees from 53 institutions in 12 countries, collectively known as the IceCube Collaboration. The IceCube Neutrino Observatory is internationally funded, with significant contributions from the National Fund for Scientific Research (FNRS & FWO) in Belgium.
“Detection of a particle shower at the Glashow resonance with IceCube,” IceCube Collaboration: R. Abbasi et al. Nature
Nick van Eijndhoven, radio detection lead -VUB
Professor of physics
Krijn de Vries, radar detection lead - VUB
Professor of physics
Madeleine O’Keefe, IceCube press
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