High-energy particle detection by IceCube proves 60-year-old theory

On December 6, 2016, a high-energy particle called electronic antineutrino flew to Earth from space at a speed close to the speed of light carrying 6.3 petaelectronvolts (PeV) of energy. Deep inside the ice cap at the South Pole, it shattered into an electron and produced a particle that quickly disintegrated 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 Prize-winning physicist Sheldon Glashow in 1960. With this detection, scientists provided further confirmation of the Standard Model of particle physics. He also demonstrated the ability of IceCube, which detects nearly massless particles called neutrinos using thousands of sensors embedded in Antarctic ice, to do fundamental physics. The result was published on March 10 in 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 an article in which he predicted that an antineutrino (the antimatter twin of a neutrino) could interact with an electron to produce a yet unknown particle – if the antineutrino had only 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 what Glashow and his colleagues predicted in 1960. The Glashow resonance would require a neutrino with an energy of 6.3 PeV, almost 1000 times more energetic than what CERN’s Large Hadron Collider is capable of producing. In fact, no artificial particle accelerator on Earth, present or planned, could create a neutrino with so much energy.

But what about a Natural accelerator – in space? The enormous energies of supermassive black holes in the center of galaxies and other extreme cosmic events can generate particles with energies impossible to create on Earth. Such a phenomenon was probably responsible for the antineutrino 6.3 PeV which reached IceCube in 2016.

“When Glashow was a post-doctoral fellow at Niels Bohr, he could never have imagined that his unconventional proposal to produce the W^– The boson would be produced by an antineutrino from a distant galaxy crashing into the Antarctic ice, ”says Francis Halzen, professor of physics at the University of Wisconsin-Madison, headquarters of IceCube maintenance and operations, and senior research scientist at the University of Wisconsin-Madison. ‘IceCube.

Since IceCube began fully functioning 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 particularly noteworthy because of its remarkably high energy; this is only the third event detected by IceCube with an energy greater than 5 PeV.

“This result proves the feasibility of neutrino astronomy – and the ability of IceCube to do so – which will play an important role in the future physics of multi-messenger astroparticles,” says Christian Haack, who was a graduate student at RWTH Aachen while working on this analysis. “We can now detect individual neutrino events that are unmistakably of extraterrestrial origin.”

The result also opens a new chapter in neutrino astronomy as it begins to disentangle neutrinos from antineutrinos. “As the previous measurements were not sensitive to the difference between neutrinos and antineutrinos, this result is the first direct measurement of an antineutrino component of the astrophysical neutrino flux,” explains Lu Lu, one of the main analyzers of this article, who was a postdoc at Chiba University in Japan during the analysis.

“There are a number of properties of astrophysical neutrino sources that we cannot measure, such as the physical size of the accelerator and the strength of the magnetic field in the acceleration region,” says Tianlu Yuan, assistant researcher in Wisconsin IceCube Particle Astrophysics Center. and another main analyzer. “If we can determine the neutrino-antineutrino ratio, we can directly study these properties.”

To confirm the detection and make a decisive measurement of the neutrino-antineutrino ratio, the IceCube collaboration wants to see more Glashow resonances. A proposed extension of the IceCube detector, IceCube-Gen2, would allow scientists to perform such measurements in statistically significant ways. The collaboration recently announced a detector upgrade that will be implemented over the next few years, the first step towards IceCube-Gen2.

Glashow, now professor emeritus of physics at Boston University, echoes the need for more detections of Glashow resonance events. “To be absolutely sure, we should witness another such event at the same energy as the one that was seen,” he said. “So far there is one, and someday there will be more.”

Finally, the result demonstrates the value of international collaboration. IceCube is operated by more than 400 scientists, engineers and staff from 53 institutions in 12 countries, together known as the IceCube Collaboration. The main reviewers for this article worked together in Asia, North America, and Europe.

“The detection of this event is another ‘first’, once again demonstrating IceCube’s ability to deliver unique and exceptional results,” says Olga Botner, professor of physics at Uppsala University in Sweden and former spokesperson for the IceCube collaboration.

“IceCube is a wonderful project. Within a few years of operation, the detector discovered what it had been funded to discover: the highest energy cosmic neutrinos, their potential source in blazars, and their ability to help the multi-messenger astrophysics, “says Vladimir Papitashvili, program officer in the office of polar programs at the National Science Foundation, IceCube’s main funder. James Whitmore, program manager in the NSF Physics Division, adds: “Now IceCube amazes scientists with a rich source of new treasures that even theorists never expected to find so soon.”