  Topological Dirac and Weyl semimetals not only host quasiparticles analogous
to the elementary fermionic particles in high-energy physics, but also have
nontrivial band topology manifested by exotic Fermi arcs on the surface. Recent
advances suggest new types of topological semimetals, in which spatial
symmetries protect gapless electronic excitations without high-energy analogy.
Here we observe triply-degenerate nodal points (TPs) near the Fermi level of
WC, in which the low-energy quasiparticles are described as three-component
fermions distinct from Dirac and Weyl fermions. We further observe the surface
states whose constant energy contours are pairs of Fermi arcs connecting the
surface projection of the TPs, proving the nontrivial topology of the newly
identified semimetal state.
