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With the IceCube Laboratory visible in the distance, UD physicist Frank Schroeder shovels a trench for cables leading
to a radio antenna he deployed in January 2020.
electron antineutrino races to Earth from outer space at nearly the
speed of light, carrying 6.3 petaelectronvolts (PeV) of energy. Deep in
the ice sheet at the South Pole, this high-energy particle smashes into
an electron and produces another particle, which quickly decays into a
shower of secondary particles. The explosion of light from the collision
does not go unnoticed. It is captured by a massive telescope buried in
the Antarctic glacier, the IceCube Neutrino Observatory.
What IceCube saw on Dec. 8, 2016, was a Glashow resonance event, a
phenomenon first predicted in 1960 by Nobel Laureate Sheldon Glashow,
but that had not been previously observed. The finding, now presented by
the international IceCube collaboration, which includes researchers
from the University of Delaware, further confirms the accuracy of the
Standard Model of particle physics, which describes the fundamental
forces and building blocks of the universe. The result is published in
the March 10 issue of Nature.
“This finding contributes to the fabric of knowledge,” said David
Seckel, professor of physics and astronomy at the University of Delaware
and team lead for about a dozen scientists from UD’s Bartol Research
Institute involved in the study. “It had been speculated for some time,
but we couldn’t test it — in the particle accelerator world, we can’t
achieve such extremely high energies or unusual scenarios. That’s just
one of the areas where IceCube has contributed so significantly to
Move this whole section up, swapping places with the section above it.
Visualization of the Glashow event, nicknamed "Hydrangea," 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.
In 1960, when he was a postdoctoral researcher at what is today the
Niels Bohr Institute in Copenhagen, Denmark, Sheldon Glashow 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.
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,” said IceCube principal investigator
Francis Halzen, professor of physics at the University of
Schematic of the IceCube telescope embedded over a mile deep in
the Antarctic ice sheet at the South Pole. It includes 86 strings
holding 5,160 light sensors.
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.
“The W– boson decays into a couple of quarks, and those
quarks dress themselves up in high-energy pions and protons. That’s why
we know the energy to high precision,” Seckel said.
While other particle events give scientists the ability to more
closely track them to faraway galaxies, “contained” energy events like
this one generally don’t offer such good pointing information.
“We can track the source to about a half a degree,” Seckel said.
“Now, imagine the whole night sky. A half-degree can cover not only the
moon, but the galaxies behind it. So there’s typically a lot of sources
to take into account. But what if we could correlate what we find with
gravitational wave experiments that have revealed colliding black holes?
That’s where we are heading, with multi-messenger astronomy, where we
can link up observations of a single object or event by different
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,” said Lu Lu,
one of the main analyzers of this paper, who was a postdoc at Chiba
University in Japan during the analysis. Professor Lu will present a
colloquium to the UD Department of Physics and Astronomy on April 28.
IceCube is located at the South Pole, which remains in darkness
for half the year. Those brave enough to live at the Pole during the
long, sunless winter are rewarded with stunning views of the night sky,
including the aurora australis, or “Southern Lights.” The building
illuminated in red in the lower left is the IceCube Laboratory.
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,
echoed 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 said. “So far there’s one, and
someday there will be more.”
IceCube is operated by over 400 scientists, engineers, and staff from
53 institutions in 12 countries, together known as the IceCube
“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,” said 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, added,
“Now, IceCube amazes scientists with a rich fount of new treasures that
even theorists weren’t expecting to be found so soon.”
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 at UW–Madison. The University of Delaware receives
additional support through NSF EPSCoR (2019597). IceCube’s research
efforts 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.
Article by Tracey Bryant, adapted from IceCube Collaboration
Photos courtesy of Frank Schroeder and IceCube Collaboration
Published March 10, 2021