NASA gives go-ahead for $20M multi-institution balloon experiment led
by UChicago scientists
Sometimes a question is so big that it takes a continent to answer it.
University of Chicago physicist Abby Vieregg is leading an international
experiment that essentially uses the ice in Antarctica as a giant detector
to find extremely energetic particles from outer space. Recently approved by
NASA, the $20 million project will build an instrument to fly above the
Antarctic in a balloon, launching in December 2024.
“We are searching for the very highest-energy neutrinos in the universe,”
said Vieregg, an associate professor in the Department of Physics. “They are
made in the most energetic and extreme places in the cosmos, and these
neutrinos offer a unique glimpse into these places. Finding one or several
of them could let us learn completely new things about the universe.”
The 12-institution international collaboration will build a radio detector
attached to a high-altitude balloon, which will be launched by NASA and
travel over Antarctica at 120,000 feet, searching for signals from
neutrinos. The groundbreaking project is called PUEO, short for the Payload
for Ultrahigh Energy Observations. (It shares its name with the only living
owl native to Hawaii, where PUEO’s predecessor experiment was born.)
‘A beautiful way to look at the universe’
Neutrinos are often called “ghost” particles because they very rarely
interact with matter. Trillions pass harmlessly through your body every
second.
Because they can travel huge distances without getting distorted or
sidetracked, neutrinos can serve as unique clues about what’s happening
elsewhere in the universe—including the cosmic collisions, galaxies and
black holes where they are created.
“Neutrinos are a beautiful way to look at the universe, because they travel
unimpeded across space,” said Vieregg. “They can come from very far away,
and they don’t get scrambled along the way, so they point back to where they
came from.”
Scientists have detected a few such neutrinos from outer space coming into
the Earth’s atmosphere. But they think there are even more neutrinos out
there which carry extraordinarily high energies—several orders of magnitude
higher than even the particles being accelerated at the Large Hadron
Collider in Europe—and have never yet been detected. These neutrinos could
tell us about the most extreme events in the universe.
That is, if you can catch them.
These neutrinos so rarely interact with other forms of matter that Vieregg
would have to build an enormous, country-sized detector to catch them. Or
she can use one that already exists: the sheet of ice atop Antarctica.
“The ice cap is perfect—a homogeneous, dense, radiotransparent block that
spans millions of square kilometers,” said Vieregg. “It’s almost like we
designed it.”
Neutrinos can serve as unique clues about what’s happening elsewhere in the universe.
If one of these highly energetic neutrinos comes through the Earth, there’s
a chance it will bump into one of the atoms inside the Antarctic ice sheet.
This collision produces radio waves which pass through the ice. This radio
signal is what PUEO would detect as it floats above Antarctica.
To do so, it needs some very, very special equipment.
The next generation
PUEO is the next generation of a mission called ANITA, based out of the
University of Hawaii, which flew over the Antarctic aboard NASA balloons
four times between 2006 and 2016 to look for similar neutrinos. PUEO,
however, will have a much more powerful detector.
The new detector taps into the power of an old astronomy trick—a technique
called interferometry, which combines signals from multiple telescopes. PUEO
is studded all over with radio antennas, and a central data acquisition
system will merge and analyze these signals to make a stronger signal.
A stronger signal would be a significant leap forward, because it would help
scientists pick out the important signals from the noise washing in from all
directions. “There are terabytes of data coming into the detector every
minute, and we expect at most a few events out of billions to be a
neutrino,” said Cosmin Deaconu, a UChicago research scientist who is working
on the software for PUEO. “You can’t write all of that data to disk, so we
have to design a program to decide very quickly which signals to keep and
which to discard.”
Many common signals look like neutrinos, but aren’t. Those can range from
satellite transmissions to someone flicking a cigarette lighter. “At least
in Antarctica, there are only a few locations where humans would be
generating these, so it’s easier to rule those out,” said Deaconu. “But we
even need to account for things like static electricity, generated by wind.”
Vieregg and the team tested the idea of the interferometric phased array on
the ground in two experiments: one called ARA at the South Pole in 2018, and
another called RNO-G in Greenland in the summer of 2021. Both showed a
significant jump in performance over previous designs—which makes PUEO’s
aerial detector all the more promising. “PUEO will have a factor of 10
better sensitivity than all previous flights of ANITA combined,” said
Vieregg.
"Finding one or several of these neutrinos could let us learn completely new things about the universe."-Assoc. Prof. Abby Vieregg
In the next months, the team will build prototypes for PUEO and finalize the
design. Once the layout is final, small teams at institutions around the
country will build parts of the instrument, which will then be assembled and
tested at UChicago. “For example, we want to make sure it can handle the
vacuum of near-space,” said Eric Oberla, a UChicago research scientist who
is building PUEO’s hardware. “It’s harder to dissipate heat when there’s no
air to move it away, which can be a problem for electronics, so we'll run
tests in a vacuum chamber here on campus and later in a large NASA chamber
during the instrument integration campaign.”
From there, PUEO will ship to a NASA facility in Palestine, Texas, for final
tests before being sent to the launch station in Antarctica.
Depending on the weather conditions, the detector could fly for a month or
more, collecting data and transmitting it back to the ground, where
scientists will comb through it for evidence of the first-ever high-energy
neutrino detection.
“We are delighted to have the PUEO stratospheric balloon mission included in
the inaugural group of Pioneers missions, and are looking forward to the
great science it will return,” said Michael Garcia, lead at NASA/HQ for the
Pioneers in Astrophysics Program, which is funding the experiment.
The Pioneers program allowed the scientists to “dream big,” Vieregg said.
“We could say, ‘If we could build anything we wanted to, what could we
make?’”
“It’s a discovery experiment, meaning nothing’s guaranteed,” she added. “But
all the indications say there’s something out there for us to pick up—and
even a few neutrinos would be an amazing scientific find.”
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