On December 6, 2016, a high-energy particle called an electron antineutrino hurtled to Earth from outer space at close to the speed of light carrying 6.3 petaelectronvolts (PeV) of energy. Deep inside the ice sheet at the South Pole, it smashed into an electron and produced a particle called W– boson, which quickly decayed into a cascade of high-energy particles (a particle shower). The interaction was captured by the IceCube Neutrino Observatory, a cubic-kilometer-scale telescope that detects neutrinos using thousands of sensors embedded in the Antarctic ice. It is the first observation of a ‘Glashow resonance event,’ a phenomenon first proposed by Nobel laureate physicist Sheldon Glashow in 1960, and suggests the presence of electron antineutrinos in the astrophysical flux, while also providing further validation of the Standard Model of particle physics.
A visualization of the Glashow resonance event detected by IceCube. Image credit: IceCube Collaboration.
Sheldon Glashow, then a postdoctoral researcher at today’s Niels Bohr Institute, predicted that an antineutrino could interact with an electron to produce a then-undiscovered particle through a process known as resonant production.
When the proposed particle, the W– boson, finally was discovered in 1983, it turned out to be much heavier than what Glashow and his colleagues had expected back in 1960.
Production of the W– boson through Glashow resonance would therefore require a neutrino with an energy of 6.3 petaelectronvolts (PeV), almost 1,000 times more energetic than what CERN’s Large Hadron Collider is capable of producing.
Such a phenomenon was likely responsible for the 6.3 PeV antineutrino that reached IceCube in 2016, with an energy large enough to interact via the predicted Glashow resonance.
“When Glashow was a postdoc at Niels Bohr, he could never have imagined that his unconventional proposal for producing the W– boson would be realized by an antineutrino from a faraway galaxy crashing into Antarctic ice,” said IceCube principal investigator Professor Francis Halzen, a researcher at the University of Wisconsin-Madison.
But the Glashow resonance event is especially noteworthy because of its remarkably high energy; it is only the third event detected by IceCube with an energy greater than 5 PeV.
“The observation of this event demonstrates that the Standard Model of particle physics, which describes the fundamental forces in the Universe, holds even at extremely high energies, and also demonstrates the unique capabilities of IceCube in exploring fundamental particle physics,” said Professor Doug Cowen, a researcher at Pennsylvania State University and a member of the IceCube Collaboration.
“Finding it wasn’t necessarily a surprise, but that doesn’t mean we weren’t very happy to see it,” said Dr. Claudio Kopper, a researcher at Michigan State University and a member of the IceCube Collaboration.
“We now can detect individual neutrino events that are unmistakably of extraterrestrial origin.”
The electron antineutrino that created the Glashow resonance event traveled quite a distance before reaching IceCube. Image credit: IceCube Collaboration.
The result also opens up a new chapter of neutrino astronomy because it starts to disentangle neutrinos from antineutrinos, which until now had been indistinguishable.
This is the first direct measurement of an antineutrino component of the astrophysical neutrino flux.
“This result proves the feasibility of neutrino astronomy — and IceCube’s ability to do it — which will play an important role in future multimessenger astroparticle physics,” said Dr. Christian Haack, a researcher at the Physikalisches Institut at RWTH Aachen University and a member of the IceCube Collaboration.
“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,” said Dr. Lu Lu, a researcher at Chiba University and a member of the IceCube Collaboration.
Several questions remain, however, about the astronomical source of the antineutrino detected in 2016.
“There are a number of properties of an astrophysical neutrino’s sources that we cannot measure, like the physical size of the accelerator and the magnetic field strength in the acceleration region,” said Dr. Tianlu Yuan, a researcher at the University of Wisconsin-Madison and a member of the IceCube Collaboration.
“If we can determine the neutrino-to-antineutrino ratio, we can directly investigate these properties.”
The results were published in the March 11, 2021 issue of the journal Nature.
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M.G. Aartsen et al. (The IceCube Collaboration). 2021. Detection of a particle shower at the Glashow resonance with IceCube. Nature 591, 220-224; doi: 10.1038/s41586-021-03256-1
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