Science - USA (2021-07-16)

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266 16 JULY 2021 • VOL 373 ISSUE 6552 SCIENCE



igh on Greenland’s ice sheet, research-
ers are drilling boreholes this week.
But they are not earth scientists seek-
ing clues to the past climate. They are
particle astrophysicists, searching for
the cosmic accelerators responsible
for the universe’s most energetic particles.
By placing hundreds of radio antennas on
the ice surface and dozens of meters below
it, they hope to trap elusive particles known
as neutrinos at higher energies than ever be-
fore. “It’s a discovery machine, looking for the
first neutrinos at these energies,” says Cosmin
Deaconu of the University of Chicago, speak-
ing from Greenland’s Summit Station.
Detectors elsewhere on Earth occasion-
ally register the arrival of ultra–high-energy
(UHE) cosmic rays, atomic nuclei that slam
into the atmosphere at speeds so high that a
single particle can pack as much energy as a
well-hit tennis ball. Researchers want to pin-
point their sources, but because the nuclei
are charged, magnetic fields in space bend
their paths, obscuring their origins.
That’s where neutrinos come in. Theorists
believe that as UHE cosmic rays set out from
their sources, they spawn so-called cosmo-
genic neutrinos as they collide with photons
from the cosmic microwave background,
which pervades the universe. Because they

are not charged, the neutrinos travel to Earth
as straight as an arrow. The difficulty comes
in catching them. Neutrinos are notoriously
reluctant to interact with matter, which al-
lows trillions to pass through you every sec-
ond without any notice. Huge volumes of
material have to be monitored to capture just
a handful of neutrinos colliding with atoms.
The largest such detector is the IceCube
Neutrino Observatory in Antarctica, which
watches for flashes of light from neutrino-
atom collisions across 1 cubic kilometer of ice
beneath the South Pole. Since 2010, IceCube
has detected many deep space neutrinos, but
only a handful—with nicknames including
Bert, Ernie, and Big Bird—that have energies
approaching 10 petaelectronvolts (PeV), the
expected energy of cosmogenic neutrinos,
says Olga Botner, an IceCube team mem-
ber at Uppsala University. “To detect several
neutrinos with even higher energies within a
reasonable time, we need to monitor vastly
larger volumes of ice.”
One way to do that is to take advantage
of another signal generated by a neutrino
impact: a pulse of radio waves. Because
the waves travel up to 1 kilometer within
ice, a widely spaced array of radio anten-
nas near the surface can monitor a much
larger volume of ice, at a lower cost, than
IceCube, with its long strings of photon de-
tectors deep in the ice. The Radio Neutrino

Observatory Greenland (RNO-G), led by the
University of Chicago, the Free University of
Brussels, and the German accelerator cen-
ter DESY, is the first concerted effort to test
the concept. When complete in 2023, it will
have 35 stations, each comprising two dozen
antennas, covering a total area of 40 square
kilometers. The team installed the first sta-
tion last week near the U.S.-run Summit
Station, at the apex of the Greenland Ice
Sheet, and has moved on to its second. The
environment is remote and unforgiving. “If
you didn’t bring something you can’t get it
shipped quickly,” Deaconu says. “You have
to make do with what you have.”
The cosmogenic neutrinos the team
hopes to capture are thought to emanate
from violent cosmic engines. The most
likely power sources are supermassive black
holes that gorge on material from their sur-
rounding galaxies. IceCube has traced two
deep space neutrinos with energies lower
than Bert, Ernie, and Big Bird to galaxies
with massive black holes—a sign they are
on the right track (Science, 26 February,
p. 872). But many more neutrinos at higher
energies are needed to confirm the link.
In addition to pinpointing the sources
of UHE cosmic rays, researchers hope the
neutrinos will show what those particles are
made of. Two major instruments that detect
UHE cosmic rays differ over their composi-
tion. Data from the Telescope Array in Utah
suggest they are exclusively protons, whereas
the Pierre Auger Observatory in Argentina
suggests heavier nuclei are mixed among the
protons. The energy spectrum of the neutri-
nos spawned by those particles should differ
depending on their composition—which in
turn could offer clues to how and where they
are accelerated.
RNO-G just might catch enough neu-
trinos to reveal those telltale energy dif-
ferences, says Anna Nelles of Friedrich
Alexander University Erlangen-Nürnberg,
one of the project leaders, who estimates
that RNO-G might catch as many as three
cosmogenic neutrinos per year. But, “If
we’re unlucky,” she says, detections might
be so scarce that scoring just one would
take tens of thousands of years.
Even if RNO-G proves to be a waiting
game, it is also a testbed for a much larger
radio array, spread over 500 square kilo-
meters, planned as part of an IceCube up-
grade. If cosmogenic neutrinos are out
there, the second generation IceCube will
find them and resolve the question of what
they are. “It could be flooded with neutri-
nos, 10 per hour,” Nelles says. “But we have
to be lucky.” j

Astronomers lay high-energy

particle traps in Greenland’s ice

Deep-space neutrinos caught by buried radio antennas

could point to powerful cosmic accelerators


Flags mark the locations of antennas designed to
detect radio pulses from neutrino collisions in the ice.

By Daniel Clery

0716NewsInDepth.indd 266 7/13/21 5:52 PM

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