Symmetry dictated universal helicity transport of Dirac fermions

Helicity is a fundamental property of Dirac fermions. Yet, the general rule of how it changes in transport remains elusive. We uncover, theoretically, the spinor state transformation and consequently universal helicity redistribution rule in two transport scenarios, e.g., through potentials of electrostatic and mass types, respectively. The former is dictated by Lorentz boost and its complex counterpart in Klein tunneling regime, which establishes miraculously a unified yet latent connection between helicity, Klein tunneling, and Lorentz boost; while the latter is governed by an abstract rotation group. The redistribution rule can be implemented in known Dirac materials.Our results provide a basis for unified understanding of helicity transport, and pave the way for exotic helicity-based applications in mesoscopic systems.