IceCube Detection of a High-Energy Particle – Antineutrino “Unmistakably of Extraterrestrial Origin”

IceCube Neutrino Event: Hydrangea

A visualization of the Glashow event recorded by the IceCube detector. Each colored circle shows an IceCube sensor that was triggered by the event; red circles indicate sensors triggered earlier in time, and green-blue circles indicate sensors triggered later. This event was nicknamed “Hydrangea.” Credit: IceCube Collaboration

The South Pole neutrino detector saw a Glashow resonance event, a phenomenon predicted by Nobel laureate 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 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 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 now can detect individual neutrino events that are unmistakably of extraterrestrial origin.” — Christian Haack

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 IceCube, which detects nearly massless particles called neutrinos using thousands of sensors embedded in the Antarctic ice, to do fundamental physics. The result was published today (March 10, 2021) in Nature.

Sheldon Glashow first proposed this resonance in 1960 when he was a postdoctoral researcher at what is today 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 interact with an electron to produce an as-yet undiscovered particle — if the antineutrino had just 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 had expected back in 1960. The Glashow resonance would require a neutrino with an energy of 6.3 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.

IceCube Neutrino Observatory Schematic

A schematic of the in-ice portion of IceCube, which includes 86 strings holding 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 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 for producing the W boson would be realized by an antineutrino from a faraway galaxy crashing into Antarctic ice,” says Francis Halzen, professor of physics at the University of Wisconsin-Madison, the headquarters of IceCube maintenance and operations, and principal investigator of IceCube.

Since IceCube started full operation in May 2011, the observatory has detected hundreds of high-energy astrophysical neutrinos 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.

“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,” says Christian Haack, who was a graduate student at RWTH Aachen while working on this analysis. “We now can detect individual neutrino events that are unmistakably of extraterrestrial origin.”

Antineutrino Journey

The electron antineutrino that created the Glashow resonance event traveled quite a distance before reaching IceCube. This graphic shows its journey; the blue dotted line is the antineutrino’s path. (Not to scale.) Credit: IceCube Collaboration

The result also opens up a new chapter of neutrino astronomy because it starts to disentangle 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 of this paper, who was a postdoc at Chiba University in Japan during the analysis.

“There are a number of properties of the astrophysical neutrinos’ sources that we cannot measure, like the physical size of the accelerator and the magnetic field strength in the acceleration region,” says Tianlu Yuan, an assistant scientist at the Wisconsin IceCube Particle Astrophysics Center and another main analyzer. “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 wants to see more Glashow resonances. 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.

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

Last but not least, the result demonstrates the value of international collaboration. IceCube is operated by over 400 scientists, engineers, and staff from 53 institutions in 12 countries, together known as the IceCube Collaboration. The main analyzers on this paper worked together across Asia, North America, and Europe.

“The detection of this event is another ‘first,’ demonstrating yet again IceCube’s capacity to deliver unique and outstanding 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 just a few years of operation, the detector discovered what it was funded to discover — the highest energy cosmic neutrinos, their potential source in blazars, and their ability to aid in multimessenger astrophysics,” says Vladimir Papitashvili, program officer in the Office of Polar Programs of the National Science Foundation, IceCube’s primary funder. James Whitmore, program officer in NSF Division of Physics, adds, “Now, IceCube amazes scientists with a rich fount of new treasures that even theorists weren’t expecting to be found so soon.”

Reference: “Detection of a particle shower at the Glashow resonance with IceCube” by The IceCube Collaboration, 10 March 2021, Nature.
DOI: 10.1038/s41586-021-03256-1

The IceCube Neutrino Observatory is funded primarily by the National Science Foundation (OPP-1600823 and PHY-1913607) and is headquartered at the Wisconsin IceCube Particle Astrophysics Center, a research center of UW-Madison in the United States. 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 University of Wisconsin-Madison Research Fund in the U.S.

12 Comments on "IceCube Detection of a High-Energy Particle – Antineutrino “Unmistakably of Extraterrestrial Origin”"

  1. Wow!

    I am as impressed as the next average Joe on the transporter, but how much knowledge are we prepared to deal with? We are seeing the most basic of building blocks of matter and are on the verge of understanding a little bit more of how sub-atomic particles interact. How awesome is that? Now consider how hard it is to find a vaccination for a serious virus here on Earth. Just sayin’. Just a little of this genius power would go a long ways here on the streets.

    • Torbjörn Larsson | March 11, 2021 at 3:42 pm | Reply

      Actually the vaccines were developed much faster than usual. Especially the lipid encased new mRNA vaccines, that apparently had 30 years of lipid vesicle research before they were ready “just in time”.

  2. Stuey Ogilvy | March 10, 2021 at 2:36 pm | Reply

    Can anyone explain that in about 4 sentences or less. I kinda getting out of this: physics guy’s theory proves right. So, these almost massless particals on earth have only so much energy. But his flux capacitor detected one with so much energy that it must be from deep space.

    Am I close?

    • Torbjörn Larsson | March 11, 2021 at 3:47 pm | Reply

      IceCube is a series of light detectors lowered into drill holes in clear glacial ice and frozen in place. They see light events from particles, both primary from searched for neutrinos (hence “IceCube neutrino detector”) and secondary ones produced by the first event.

      The event signature from this one was of an high energy antineutrino produced by a special astrophysical mechanism.

      4th sentence: the term “his flux capacitor” is demeaning both to the tax payer investment and the huge collaborations of scientists that made the observation possible.

  3. These super expensive colliders aren’t necessary. All you need is the detectors, and a quiet stable place to put the detector, and let Mother Nature do the heavy work.
    Hey, if we want to spend some money, let’s try this. Get a spacecraft that can go to Jupiter, Equip the spacecraft with particle detectors of all types. Surround the detectors with plates made of 6 different materials that act as targets. Send the spacecraft into orbit around Jupiter, and place the orbit right in the middle of Jupiter’s hellacious radiation belts. Now just record what goes on in that detector. Orient the space craft such that the material target plates are oriented in the direction of travel. Then this material becomes the target. This would be a particle accelerator detector in a space craft, and we let Jupiter be the Cyclotron. Just go into orbit in the radiation belt, and you will have more data thatn you will know what to do with it.

    • Torbjörn Larsson | March 11, 2021 at 3:52 pm | Reply

      Colliders are complementary, since they have much higher fluxes – this experiment takes years to see individual events, while accelerators see billions of events *every second*.

      Accellerator particles needs humongous detectors – say, ATLAS mass 7,000 mt – and computer farms – “the LHC Computing Grid is the world’s largest computing grid comprising over 170 computing facilities in a worldwide network across 42 countries”.

      Let the experts be experts – they too want as much science as possible out of tax investments.

  4. Mark A. O'Blazney | March 11, 2021 at 10:12 am | Reply

    Ice Cube ? Wasn’t that, like, a rapper and stuff? What of ice cubed? find out in the radiation belt+

  5. Torbjörn Larsson | March 11, 2021 at 3:40 pm | Reply

    The event is just 2.3 sigma, far from detection, see more here: https://www.nature.com/articles/d41586-021-00486-1 .

    But it is promising:

    “Nevertheless, the IceCube Collaboration’s observations are cause for celebration, because they are the first to be consistent with a Glashow resonance. An unambiguous detection would not only provide further confirmation of the standard model of particle physics, but also prove that electron antineutrinos are present in astrophysical fluxes.”

    “Future measurements of electron antineutrinos will therefore open a window on the physics of neutrino sources. The current IceCube detector can detect only a low number of Glashow-resonance events, but the next generation of the apparatus — IceCube-Gen2 — was proposed last year7. This detector will have a sufficiently large volume of sensor-probed ice to observe higher numbers of Glashow-resonance events, thus enabling a statistically meaningful analysis of astrophysical neutrino-production mechanisms.”

  6. Ed,
    Physicists are a smart bunch, but they usually are not double-major epidemiologists.
    We found the vaccine. Please take it when it’s your turn.

  7. … The Physics are just in transition toward a theory that will work, this worlds are not working in a reasonable way…

  8. I put ice cubes in my whiskey.

  9. Stephen Scrutton | March 18, 2021 at 7:09 pm | Reply

    To quote from the article:

    “The result also opens up a new chapter of neutrino astronomy because it starts to disentangle neutrinos from antineutrinos.”

    Can this be considered evidence that neutrinos follow Dirac statistics rather than Majorana statistics? Majorana, of course, states that a neutrino is its own antiparticle.

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