UNIVERSITY PARK, Pa. — An incredibly high-energy cosmic particle called an electron antineutrino — the electron neutrino’s antimatter twin — has been directly observed on Earth for the first time by the IceCube collaboration, an international team that includes Penn State physicists. This also marks the first observation of "Glashow resonance," a phenomenon predicted in 1960, and serves as a unique test of the Standard Model of particle physics at extremely high energies.

The antineutrino approached Earth from outer space on Dec. 8, 2016, at close to the speed of light. Deep inside the ice sheet that covers the South Pole, it smashed into an electron in the ice and produced a heavy charged particle that quickly decayed into a shower of secondary particles. The interaction was captured by the IceCube Neutrino Observatory, a cubic-kilometer-scale telescope that detects neutrinos — the elusive, nearly massless sub-atomic particles that flood the universe — using thousands of sensors embedded in the Antarctic ice. The discovery appears March 10 in the journal Nature.

“This interaction of an antineutrino and an electron is an example of a Glashow resonance event, which was first proposed by Nobel laureate physicist Sheldon Glashow in 1960,” said Doug Cowen, professor of physics at Penn State and a member of the collaboration. “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.”

Glashow, then a postdoctoral researcher at today's Niels Bohr Institute in Copenhagen, Denmark, predicted that an antineutrino could interact with an electron to produce a then-undiscovered particle — if the antineutrino had just the right energy — 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. In fact, no human-made particle accelerator on Earth, current or planned, could create a neutrino with that 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, 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 Francis Halzen, professor of physics at the University of Wisconsin–Madison and principal investigator of IceCube.

Since IceCube started full operation in May 2011, the observatory has detected hundreds of high-energy neutrinos of astrophysical origin and has produced a number of significant results in particle astrophysics, including the discovery of an astrophysical neutrino flux in 2013 and the first identification of a source of astrophysical neutrinos in 2018. 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 light produced by a Glashow-resonance antineutrino interacting in the ice extends hundreds of meters from the neutrino interaction point,” said Cowen. “Only a detector the size of IceCube is capable of containing, and accurately measuring, such prodigious light emission.”

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. 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 Tianlu Yuan, an assistant scientist at the University of Wisconsin–Madison, and one of the main analyzers of the paper. “If we can determine the neutrino-to-antineutrino ratio, we can directly investigate these properties.”

To confirm the detection and make a decisive measurement of the neutrino-to-antineutrino ratio, the IceCube Collaboration hopes to observe more Glashow resonance interactions. A proposed expansion of the IceCube detector, IceCube-Gen2, would enable the scientists to make such measurements in a statistically significant way. The collaboration recently announced an upgrade of the detector that will be implemented over the next few years, the first step toward IceCube-Gen2.

“With its unique capability to detect ultra-high energy neutrino interactions, the IceCube experiment will continue exploring entirely new realms in the neutrino landscape, and more discoveries may be just around the corner," said Cowen.

The IceCube Neutrino Observatory is funded primarily by the National Science Foundation and is headquartered at the University of Wisconsin–Madison. IceCube’s research efforts, including critical contributions to the detector operation, are funded by agencies in Australia, Belgium, Canada, Denmark, Germany, Japan, New Zealand, Republic of Korea, Sweden, Switzerland, the United Kingdom, and the United States. IceCube construction was also funded with significant contributions from the National Fund for Scientific Research (FNRS & FWO) in Belgium; the Federal Ministry of Education and Research (BMBF) and the German Research Foundation (DFG) in Germany; the Knut and Alice Wallenberg Foundation, the Swedish Polar Research Secretariat, and the Swedish Research Council in Sweden; and the Department of Energy and the University of Wisconsin–Madison Research Fund in the United States. The IceCube EPSCoR Initiative also receives additional support from the National Science Foundation.