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Mysterious particles spewing from Antarctica defy physics

By [31]Rafi Letzter 2020-01-26T19:05:58Z

What's making these things fly out of the frozen continent?
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Researchers prepare to launch the Antarctic Impulsive Transient Antenna
(ANITA) experiment, which picked up signals of impossible-seeming
particles as it dangled from its balloon over Antarctica.
Researchers prepare to launch the Antarctic Impulsive Transient Antenna
(ANITA) experiment, which picked up signals of impossible-seeming
particles as it dangled from its balloon over Antarctica.
(Image: © NASA)

Our best model of particle physics is bursting at the seams as it
struggles to contain all the weirdness in the universe. Now, it seems
more likely than ever that it might pop, thanks to a series of strange
events in Antarctica. .

The death of this reigning physics paradigm, the Standard Model, has
been predicted for decades. There are hints of its problems in the
physics we already have. Strange results from laboratory experiments
suggest flickers of [32]ghostly new species of neutrinos beyond the
three described in the Standard Model. And the universe seems [33]full
of dark matter that no particle in the Standard Model can explain.

But recent tantalizing evidence might one day tie those vague strands
of data together: Three times since 2016, ultra-high-energy particles
have blasted up through the ice of Antarctica, setting off detectors in
the Antarctic Impulsive Transient Antenna (ANITA) experiment, a machine
dangling from a NASA balloon far above the frozen surface.

Related:[34] The 18 Biggest Unsolved Mysteries in Physics

As Live Science [35]reported in 2018, those events — along with several
additional particles detected later at the buried Antarctic neutrino
observatory IceCube — don't match the expected behavior of any
[36]Standard Model particles. The particles look like ultra high-energy
[37]neutrinos. But ultra high-energy neutrinos shouldn't be able to
pass through the Earth. That suggests that some other kind of particle
— one that's never been seen before — is flinging itself into the cold
southern sky.

Now, in a new paper, a team of physicists working on IceCube have cast
heavy doubt on one of the last remaining Standard Model explanations
for these particles: cosmic accelerators, giant neutrino guns hiding in
space that would periodically fire intense neutrino bullets at Earth. A
collection of hyperactive neutrino guns somewhere in our northern sky
could have blasted enough neutrinos into Earth that we'd detect
particles shooting out of the southern tip of our planet. But the
IceCube researchers didn't find any evidence of that collection out
there, which suggests new physics must be needed to explain the
mysterious particles.

To understand why, it's important to know why these mystery particles
are so unsettling for the Standard Model.

Neutrinos are the faintest particles we know about; they're difficult
to detect and nearly massless. They pass through our planet all the
time — mostly coming from the sun and rarely, if ever, colliding with
the protons, neutrons and electrons that make up our bodies and the
dirt beneath our feet.

But ultra-high-energy neutrinos from deep space are different from
[38]their low-energy cousins. Much rarer than low-energy neutrinos,
they have wider "cross sections," meaning they're more likely to
collide with other particles as they pass through them. The odds of an
ultra-high-energy neutrino making it all the way through Earth intact
are so low that you'd never expect to detect it happening. That's why
the ANITA detections were so surprising: It was as if the instrument
had won the lottery twice, and then IceCube had won it a couple more
times as soon as it started buying tickets.

And physicists know how many lottery tickets they had to work with.
Many ultra-high-energy cosmic neutrinos come from the interactions of
cosmic rays with the cosmic microwave background (CMB), the faint
afterglow of the Big Bang. Every once in a while, those cosmic rays
interact with the CMB in just the right way to fire high-energy
particles at Earth. This is called the "flux," and it's the same all
over the sky. Both ANITA and IceCube have already measured what the
cosmic neutrino flux looks like to each of their sensors, and it just
doesn't produce enough high-energy neutrinos that you'd expect to
detect a neutrino flying out of Earth at either detector even once.

"If the events detected by ANITA belong to this diffuse neutrino
component, ANITA should have measured many other events at other
elevation angles," said Anastasia Barbano, a University of Geneva
physicist who works on IceCube.

But in theory, there could have been ultra-high-energy neutrino
sources beyond the sky-wide flux, Barbano told Live Science: those
neutrino guns, or cosmic accelerators.

Related: [39]The 11 Biggest Unanswered Questions About Dark Matter

"If it is not a matter of neutrinos produced by the interaction of
ultra-high-energy cosmic rays with the CMB, then the observed events
can be either neutrinos produced by individual cosmic accelerators in a
given time interval" or some unknown Earthly source, Barbano said.

Blazars, active galactic nuclei, gamma-ray bursts, starburst galaxies,
galaxy mergers, and magnetized and fast-spinning neutron stars are all
good candidates for those sorts of accelerators, she said. And we know
that cosmic neutrino accelerators do exist in space; in 2018, IceCube
[40]tracked a high-energy neutrino back to a blazar, an intense jet of
particles coming from an active black hole at the center of a distant
galaxy.

ANITA picks up only the most extreme high-energy neutrinos, Barbano
said, and if the upward-flying particles were
cosmic-accelerator-boosted neutrinos from the Standard Model — most
likely tau neutrinos — then the beam should have come with a shower of
lower-energy particles that would have tripped IceCube's lower-energy
detectors.

"We looked for events in seven years of IceCube data," Barbano said —
events that matched the angle and length of the ANITA detections, which
you'd expect to find if there were a significant battery of cosmic
neutrino guns out there firing at Earth to produce these up-going
particles. But none turned up.

Their results don't completely eliminate the possibility of an
accelerator source out there. But they do "severely constrain" the
range of possibilities, eliminating all of the most plausible scenarios
involving cosmic accelerators and many less-plausible ones.

"The message we want to convey to the public is that a Standard Model
astrophysical explanation does not work no matter how you slice it,"
Barbano said.

Researchers don't know what's next. Neither ANITA nor IceCube is an
ideal detector for the needed follow-up searches, Barbano said, leaving
the researchers with very little data on which to base their
assumptions about these mysterious particles. It's a bit like trying to
figure out the picture on a giant jigsaw puzzle from just a handful of
pieces.

Right now, many possibilities seem to fit the limited data, including a
fourth species of "sterile" neutrino outside the Standard Model and a
range of theorized types of dark matter. Any of these explanations
would be revolutionary.hjh But none is strongly favored yet.

"We have to wait for the next generation of neutrino detectors,"
Barbano said.

The paper has not yet been peer reviewed and was [41]published January
8 in the arXiv database.
* [42]From Big Bang to Present: Snapshots of Our Universe Through
Time
* [43]The 11 Biggest Unanswered Questions About Dark Matter
* [44]5 Elusive Particles Beyond the Higgs

Originally published on [45]Live Science.

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