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An elusive hypothetical particle comes
in imitation form.

Lurking within a solid crystal is a
phenomenon that is mathematically similar to proposed subatomic particles
called axions
, physicist Johannes
Gooth and colleagues report online October 7 in Nature.

If axions exist as fundamental
particles, they could constitute a hidden form of matter in the cosmos, dark
matter. Scientists know dark matter exists thanks to its gravitational pull,
but they have yet to identify what it is. Axions are one possibility, but no one has found the particles yet (SN: 4/9/18).

Enter the imitators. The axions analogs
within the crystal are a type of quasiparticle, a disturbance in a material that
can mimic fundamental particles like axions. Quasiparticles result from the
coordinated jostling of electrons within a solid material. It’s a bit like how birds
in a flock seem to take on new forms by syncing up their movements.


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Axions were first proposed in the
context of quantum chromodynamics — the theory that explains the behaviors of quarks,
tiny particles that are contained, for example, inside protons. Axions and
their new doppelgängers “are mathematically similar but physically totally
unrelated,” says theoretical physicist Helen Quinn of SLAC National Accelerator
Laboratory in Menlo Park, Calif., one of the scientists who formulated the
theory behind axions. That means scientists are no closer to solving their dark
matter woes.

Still, the new study reveals for the
first time that the phenomenon has a life beyond mere equations, in
quasiparticle form. “It’s actually amazing,” says Gooth, of the Max Planck Institute
for Chemical Physics of Solids in Dresden, Germany. The idea of axions is “a
very mathematical concept, in a sense, but it still exists in reality.”

In the new study, the researchers
started with a material that hosts a type of quasiparticle known as a Weyl fermion,
which behaves as if massless (SN: 7/16/15).
When the material is cooled, Weyl fermions become locked into place, forming a
crystal. That results in the density of electrons varying in a regular pattern
across the material, like a stationary wave of electric charge, with peaks in
the wave corresponding to more electrons and dips corresponding to fewer
electrons.

Applying parallel electric and magnetic
fields to the crystal caused the wave to slosh back and forth. That sloshing is
the mathematical equivalent of an axion, the researchers say.

To confirm that the sloshing was
occurring, the team measured the electric current through the crystal. That
current grew quickly as the researchers ramped up the electric field’s strength,
in a way that is a fingerprint of axion quasiparticles.

If the scientists changed the direction
of the magnetic field so that it no longer aligned with the electric field, the
enhanced growth of the electric current was lost, indicating that the axion
quasiparticles went away. “This material behaves exactly as you would expect,”
Gooth says.

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