Physicists have found quasiparticles that mimic hypothetical dark matter axions

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.

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.