A visualization of the Glashow event recorded by the IceCube detector. Each colored circle shows an IceCube sensor caused by the event; red circles indicate sensors that were activated earlier in the corridor, and green-blue circles indicate that sensors were activated later. This event has the nickname “Hortensia”. Credit: IceCube Collaboration
The South Pole neutrino detector saw a resonant event of Glashow, a phenomenon predicted by Nobel Prize-winning physicist Sheldon Glashow in 1960, where an electron antineutrino and an electron interact to produce a W boson.
On December 6, 2016, a high-energy particle called an electron antineutrino is thrown from outer space into the earth at near the speed of light carrying 6.3 petaelectron volts (PeV) energy. Deep inside the ice sheet at the South Pole, it broke into an electron and produced a particle that quickly decayed into a shower of secondary particles. The interaction was captured by a massive telescope buried in the Antarctic glacier, the IceCube Neutrino Observatory.
“We can now detect individual neutrino events that are unmistakably of extraterrestrial origin.” – Christian Haack
IceCube saw a Glashow resonance event, a phenomenon predicted by Nobel Prize-winning physicist Sheldon Glashow in 1960. With this discovery, scientists once again confirmed the standard model of particle physics. It also demonstrated the ability of IceCube, which detects nearly massless particles called neutrinos by means of thousands of sensors embedded in the Antarctic ice, to do fundamental physics. The result is today (10 March 2021) in Nature.
Sheldon Glashow first proposed this resonance in 1960 when he was a postdoctoral researcher at the Niels Bohr Institute in Copenhagen, Denmark. There he wrote a paper in which he predicted that an antineutrino (a neutrino’s antimatter twin) could travel with an electron to produce an as yet undiscovered particle – as the antineutrino only the right energy – through a process called resonance.
When the proposed particle, the W– Boson, finally discovered in 1983, appears to be much heavier than Glashow and his colleagues expected in 1960. The Glashow resonance requires a neutrino with an energy of 6.3 PeV, almost 1000 times more energetic than what CERN‘s Large Hadron Collider can produce. In fact, no human particle accelerator on earth, currently or planned, can create a neutrino with so much energy.
A scheme of the in-ice section of IceCube, containing 86 strings containing 5,160 light sensors arranged in a three-dimensional hexagonal grid. Credit: IceCube Collaboration
But what about a natural accelerator – in space? The enormous energies of supermassive black holes in the centers of galaxies and other extreme cosmic events can generate particles with energies that are impossible to create on earth. Such a phenomenon was probably responsible for the 6.3 PeV antineutrino that IceCube reached in 2016.
‘When Glashow was a postdoctoral fellow with Niels Bohr, he could never have imagined that his unconventional proposal to the W– boson would be realized by an antineutrino from a distant galaxy crashing against the ice of Antarctica, ‘says Francis Halzen, professor of physics at the University of Wisconsin-Madison, the headquarters of IceCube maintenance and operations, and chief investigator from IceCube.
Since IceCube went into full operation in May 2011, the observatory has detected hundreds of high-energy astrophysic neutrinos and produced a number of important results in particle astrophysics, including the discovery of an astrophysic neutrino flood in 2013 and the first identification of a source of astrophysic neutrinos in 2018. But the Glashow resonance event is particularly notable for its strikingly high energy; this is only the third event detected by IceCube with an energy greater than 5 PeV.
“This result demonstrates the feasibility of neutrino astronomy – and the ability of IceCube to do so – which will play an important role in future physiology with a much-discussed astro-particle,” said Christian Haack, a graduate student at RWTH Aachen. was while working on this analysis. “We can now detect individual neutrino events that are unmistakably of extraterrestrial origin.”
The electron antineutrino that created the Glashow resonance event traveled a long way before reaching IceCube. This image shows his journey; the blue dotted line is the antineutrino path. (Not to scale.) Credit: IceCube collaboration
The result also opens a new chapter of neutrino astronomy as it begins to disrupt neutrinos of antineutrinos. “Previous measurements were not sensitive to the difference between neutrinos and antineutrinos, so this result is the first direct measurement of an antineutrino component of the astrophysic neutrino flood,” said Lu Lu, one of the lead analysts of this article, who a postdoc at Chiba University in Japan during the analysis.
“There are a number of properties of the astrophysic neutrinos that we cannot measure, such as the physical size of the accelerator and the magnetic field strength in the acceleration zone,” said Tianlu Yuan, an assistant scientist at the Wisconsin IceCube Particle Astrophysics Center. and another chief analyst. “If we can determine the neutrino-antineutrino ratio, we can investigate these properties directly.”
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 enable 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 towards IceCube-Gen2.
Glashow, now an emeritus professor of physics at Boston University, affirms the need for more detection of Glashow resonance events. “To be absolutely sure, we still have to see such an event on the same energy as what was seen,” he says. “So far there is one, and one day there will be more.”
Last but not least, the result shows the value of international cooperation. IceCube is run by more than 400 scientists, engineers and staff from 53 institutions in 12 countries, collectively known as the IceCube Collaboration. The lead analysts on this article have worked together in Asia, North America and Europe.
“The detection of this event is another ‘first’, which once again proves that IceCube has the ability to deliver unique and excellent results,” says Olga Botner, professor of physics at Uppsala University in Sweden and former spokesperson for the IceCube Collaboration.
“IceCube is a wonderful project. “In a few years of operation, the detector he funded to discover – the cosmic neutrinos with the highest energy, their potential source in jackets, and their ability to help with multi-directional astrophysics,” said Vladimir Papitashvili, program officer in the office of Polar Programs of the National Science Foundation, the primary funder of IceCube, James Whitmore, program officer of the NSF Division of Physics, adds: “Now IceCube surprises scientists with a rich amount of new treasures that even theorists do not so soon would not expect. “
Reference: “Detection of a particle parent at the Glashow resonance with IceCube” 10 March 2021, Nature.
DOI: 10.1038 / s41586-021-03256-1
The IceCube Neutrino Observatory is primarily funded by the National Science Foundation (OPP-1600823 and PHY-1913607) and is headquartered at the Wisconsin IceCube Particle Astrophysics Center, a UW-Madison research center in the United States. IceCube’s research efforts, including critical contributions to the detection operation, are funded by agencies in Australia, Belgium, Canada, Denmark, Germany, Japan, New Zealand, the Republic of Korea, Sweden, Switzerland, the United Kingdom and the United States. IceCube construction has also been 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 University of Wisconsin-Madison Research Fund in the USA