Weyl fermions are exotic relativistic particles. While their realization as elementary particles in high-energy physics remains elusive, they have recently been discovered as quasiparticles that appear in so-called topological metals. A study published in Nature Physics now demonstrates how these quasiparticles can be annihilated if the crystal is inserted into a strong magnetic filed. An abrupt reversal of the sign of the material’s Hall conductivity has been identified as the fingerprint of this annihilation. The condensed matter theory group at UZH (Prof. Titus Neupert) contributed to the theoretical interpretation of this phenomenon.