Scientists have discovered a phenomenon that looks a whole lot like a dark matter particle in a laboratory system, according to new research.
Axions are a yet-to-be-observed fundamental particle that are predicted to exist by physics theory. They’re a potential explanation as to what’s causing the mysterious cosmic anomalies attributed to dark matter. But physicists have recently realised that axion-like behaviour could show up from the collective action of particles in crystals. Scientists in Germany, the United States, and China have now observed that behaviour.
“We show that axions exist at all — the mathematical concept does apply to some part of nature,” Johannes Gooth, the study’s first author from the Max Planck Institute for the Chemical Physics of Solids, told Gizmodo.
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Axions arise as a solution to something in particle physics called the strong-CP problem. Atoms are made up of protons and neutrons, which are in turn made of up subatomic particles called quarks. Quarks have an antiparticle partner, which has the same mass but opposite electrical charge. The physical rules that govern quarks work the same as they do for antiquarks if you switch “left” and “right,” as in a mirror — but there’s no reason why they should.
Scientists Roberto Peccei and Helen Quinn devised a new kind of physical field that explains why quark physics would maintain this symmetry. The field comes with a corresponding particle, called the axion. Axions would barely interact with regular matter and are therefore a potential candidate for dark matter, a mysterious mass that seems to have large-scale effects throughout the universe but no known identity.
The axion that appears in this new research is not a fundamental particle but instead is an analogue, based on the collective behaviour of lots of particles in a material called a Weyl semimetal. The collective behaviour looks similar in the material to the way a fundamental axion should look in free space. Scientists observed it based on the strange way the material responded to high electric and magnetic fields, according to the paper published in Nature.
A Weyl semimetal is a strange material. The collective behaviour of its protons and electrons make it seem as if, instead of containing electrons with mass, it has massless, right- and left-handed electrical charges moving around inside it, called Weyl fermions. It’s kind of like the way we treat ocean waves as individual units with their own height and speed, even though we know they’re actually just disturbances in the water. The Weyl fermions form a lattice of electric charge, through which the researchers observed electric disturbances travelling. These disturbances, a collective behaviour of collective behaviours — like an ocean wave made of more ocean waves — were the system’s axions.
Again, researchers haven’t discovered “real” axions but axion-like disturbances that appear in a highly engineered system. Perhaps they can use this discovery to provide constraints as to what an axion in the vacuum of space might look like.
This experiment falls into a category we cover a lot: Things that are difficult or impossible to understand in space, like the Big Bang or black holes or the specifics of Einstein’s theory of general relativity, become comprehensible in highly engineered analogs in laboratories. Weyl semimetals have proven to be an especially good system for creating these analogs.
Does this mean that we’re closer to finding dark matter? Probably not. But the work at least suggests that axions could be real.
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