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After 16 years of theoretical anticipation, physicists have finally observed a quasiparticle exhibiting mass only in one direction of motion. ︀︀ ︀︀In a study of the topological semi-metal ZrSiS, researchers employed high-field magneto-optical spectroscopy—shining infrared light on the material under intense magnetic fields—to uncover semi-Dirac fermions. These are not fundamental particles but emergent quasiparticles arising from the collective behavior of electrons in the crystal lattice. ︀︀ ︀︀In one direction, the quasiparticle disperses linearly, behaving as massless (like a photon or Dirac fermion), enabling frictionless, high-speed propagation. Perpendicular to that axis, however, the dispersion becomes quadratic, imparting effective mass and introducing inertia, which slows motion and resists acceleration. ︀︀ ︀︀The team likens this to a train gliding effortlessly along one track but suddenly feeling its weight when switching to a perpendicu…