  A second-order phase transition is associated with emergence of an "order
parameter" and a spontaneous symmetry breaking. For the heavy fermion
superconductor URu$_2$Si$_2$, the symmetry of the order parameter associated
with its ordered phase below 17.5 K has remained ambiguous despite 30 years of
research, and hence is called "hidden order" (HO). Here we use polarization
resolved Raman spectroscopy to specify the symmetry of the low energy
excitations above and below the HO transition. These excitations involve
transitions between interacting heavy uranium 5f orbitals, responsible for the
broken symmetry in the HO phase. From the symmetry analysis of the collective
mode, we determine that the HO parameter breaks local vertical and diagonal
reflection symmetries at the uranium sites, resulting in crystal field states
with distinct chiral properties, which order to a commensurate chirality
density wave ground state.
