The dark matter candidate could show filamentous effects in the lab

A hypothetical particle that could solve one of cosmology’s greatest puzzles has just gotten a little less mysterious. A RIKEN physicist and two colleagues have revealed the mathematical basis that could explain how so-called axions could generate string-like entities that create a strange tension in lab materials.

Axions were first proposed in the 1970s by physicists studying the theory of quantum chromodynamics, which describes how certain elementary particles are held together within the atomic nucleus. The problem was that this theory predicted some bizarre properties for known particles that are not observed. To solve this problem, physicists hypothesized a new particle, later dubbed the axion, after a brand of laundry detergent, because it helped clean up a mess in the theory.

Physicists soon realized that axions could also clear up a cosmic puzzle. More than 80% of the matter in the Universe is believed to be made up of a mysterious, invisible substance called dark matter. “Axions are candidates for dark matter, but we haven’t found them yet,” says Yoshimasa Hidaka of the RIKEN Interdisciplinary Theoretical and Mathematical Sciences program. Axions might have the right properties, so physicists have looked for signs of existence in numerous experiments. In June 2020, the XENON1T experiment at the Gran Sasso Laboratory in Italy reported clues that may have detected the axion, but that result has yet to be confirmed.

But there is another arena in which the properties of axions can be studied. Physicists can prepare exotic materials, called topological insulators, in the laboratory that exhibit strange properties, such as conducting electricity on their surfaces while remaining electrical insulators inside. Such materials exhibit other strange behaviors. Sometimes, their electrons cluster and move in such a way that the material appears to be made up of “quasiparticles” with unusual properties. This can create unexpected tension across the material, called an anomalous Hall effect.

Axion is expected to arise in this way, in topological insulators, where it should interact with light particles, or photons, differently from regular particles.

Hidaka and his two colleagues have now examined the theory governing the interaction between axions and photons. Although axions are point-like particles, the team calculated that within materials, light actually interacts with extended thread-like configurations made of axions, called axion strings. This would lead to the anomalous Hall effect, which is observed in experiments.

“We found the mathematical structure behind the phenomenon,” says Hidaka.