Imagine a translator working in a noisy hall. The hum of voices, the clatter of dishes, fragments of other people’s conversations: it seems they should constantly make mistakes. But by the end of the day, their notes match perfectly with what they would have written in the quiet of an office. This is roughly how the quantum rule known as the axial anomaly behaves. It says that a certain charge current can disappear even where classical physics promises it should be conserved.

The authors of the new study calculated this anomaly for matter compressed to the densities of a neutron star. The individual contributions to the rule are rearranged: the medium adds its own corrections to the current and to the particle density. But in total, these additions cancel each other out to the last digit. The final formula remains exactly the same as for empty space. Even if the particles have completely lost their mass, the medium does not spoil the result.

This result immediately clarifies the mystery of the chiral magnetic effect. If you place a fermionic medium in a magnetic field and add an axial chemical potential, a current will flow through it. Previously, there was debate about whether it would decay. The calculation showed that the current not only flows but is conserved, like water in a ring pipe. Its strength is set by the potential, but the speed of the charge carriers turns out to be lower than usual: it is limited by the Fermi velocity. This is like a traffic jam on a highway: many cars, but they move slowly.

Why does this matter? Axionic dark matter, which dozens of experiments are searching for, acts on ordinary matter precisely as such an axial chemical potential. Therefore, in a strong magnetic field, it should generate an electric current. The authors propose using this effect as a new way to hunt for these elusive particles. The circle closes: the translator in the noisy hall not only avoids mistakes: their work opens a door to the dark sector of the Universe.