In high-energy physics, Poincaré symmetry puts strong constraints on the standard model, leading to only three different types of fermions, i.e., Dirac, Weyl, and Majorana fermions. Although several kinds of particles (such as protons) are confirmed to be Dirac fermions, the signature of Weyl and Majorana fermions is still lacking in particle physics experiments. In contrast to the stagnant situation in highenergy physics, great progress has been made recently in the realization of their low-energy quasiparticles in condensed matter.
More interestingly, due to the less constraints placed by the space group (SG) symmetries, condensed matter systems can host various new types of quasiparticles without counterparts in high-energy physics, such as threefold spin-1 excitations, fourfold spin-3/2 Rarita-Schwinger-Weyl (RSW) fermions, sixfold excitations, etc. For details, see this article!