A detector of high-energy particles from space located under the ice of the South Pole has experimentally tested a physical theory from 60 years ago.
On December 6, 2016, an electron antineutrino slammed into Earth from outer space at a speed close to the speed of light carrying 6.3 petaelectronvolts (PeV) of energy. Deep in the ice sheet at the South Pole, it smashed into an electron and produced a particle that quickly decomposed into a shower of secondary particles. The interaction was captured by a massive telescope buried in the Antarctic glacier, the IceCube Neutrino Observatory.
IceCube had seen a Glashow resonance event, a phenomenon predicted by Nobel laureate physicist Sheldon Glashow in 1960. With this detection, scientists provided another confirmation of the Standard Model of particle physics. It also further demonstrated the ability of the IceCube, which detects near-massless particles called neutrinos using thousands of sensors embedded in Antarctic ice, to perform fundamental physics. The result was published months ago in the journal Nature.
Sheldon Glashow first proposed this resonance in 1960 when he was a postdoctoral researcher at what is now the Niels Bohr Institute in Copenhagen, Denmark. There, he wrote a paper in which he predicted that an antineutrino (the antimatter twin of a neutrino) could interact with an electron to produce a yet undiscovered particle, if the antineutrino had the right energy, through a process known as resonance. .
When the proposed particle, the W boson, was finally discovered in 1983, it turned out to be much heavier than Glashow and his colleagues expected in 1960. The Glashow resonance would require a neutrino with an energy of 6.3 PeV, almost 1,000 times more energetic than CERN’s Large Hadron Collider can produce. In fact, no human-made particle accelerator on Earth, current or planned, could create a neutrino with this much energy.
But what about a natural accelerator in space? The enormous energies of supermassive black holes at the centers of galaxies and other extreme cosmic events can generate particles with energies impossible to create on Earth. Such a phenomenon was likely responsible for the 6.3 PeV antineutrino that reached IceCube in 2016.
“When Glashow was a postdoc at Niels Bohr, he could never have imagined that his unconventional proposal to produce the W boson would be made by an antineutrino from a distant galaxy crashing into the ice of Antarctica,” says Francis Halzen, professor. in physics at the University of Wisconsin-Madison, the headquarters of IceCube maintenance and operations, and a principal investigator for IceCube.
Since IceCube became fully operational in May 2011, the observatory has detected hundreds of high-energy astrophysical neutrinos and produced a number of significant results in particle astrophysics, including the discovery of an astrophysical neutrino flux in 2013 and the first identification of an astrophysical neutrino source in 2018. But the Glashow resonance event is especially notable due to its remarkably high energy; it is only the third event detected by IceCube with an energy greater than 5 PeV.
“This result proves the feasibility of neutrino astronomy and IceCube’s ability to do so, which will play an important role in future multi-messenger astroparticle physics,” says Christian Haack, a graduate student at RWTH Aachen while working on this analysis. . “We can now detect single neutrino events that are unmistakably extraterrestrial in origin.”
The result also opens a new chapter in neutrino astronomy because it begins to unravel neutrinos from antineutrinos. “Previous measurements have not been sensitive to the difference between neutrinos and antineutrinos, so this result is the first direct measurement of an antineutrino component of the astrophysical neutrino flux,” says Lu Lu, one of the main analyzers for this article, who was a postdoc at Chiba University in Japan during the analysis.
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A Nobel Prize raised a theory 60 years ago and is now being tested in Antarctica – Format Seven