Topological Dirac and Weyl semimetals not only host quasiparticles analogous to the elementary fermionic particles in high-energy physics, but also have a non-trivial band topology manifested by gapless surface states, which induce exotic surface Fermi arcs^1,2. Recent advances suggest new types of topological semimetal, in which spatial symmetries protect gapless electronic excitations without high-energy analogues^{3,4,5,6,7,8,9,10,11}. Here, using angle-resolved photoemission spectroscopy, we observe triply degenerate nodal points near the Fermi level of tungsten carbide with space group \(P\bar{6}m2\) (no. 187), in which the low-energy quasiparticles are described as three-component fermions distinct from Dirac and Weyl fermions. We further observe topological surface states, whose constant-energy contours constitute pairs of ‘Fermi arcs’ connecting to the surface projections of the triply degenerate nodal points, proving the non-trivial topology of the newly identified semimetal state.

拓扑狄拉克和魏尔半金属不仅具有与高能物理中的基本费米离子粒子相似的准粒子，而且具有以平整的表面态表现出来的非平整的带拓扑结构，从而引起了奇异的表面费米弧^1,2。最近的进展提出了新型的拓扑半金属，其中空间对称性保护了无间隙的电子激发，而没有高能类似物^{3,4,5,6,7,8,9,10,11}。在这里，使用角度分辨光发射光谱，我们观察到碳化钨的费米能级附近具有空间群\（P \ bar {6} m2 \）的三重简并节点。（第187号），其中低能准粒子被描述为与狄拉克（Dirac）和魏尔（Weyl）费米子不同的三组分费米子。我们进一步观察到拓扑表面状态，其恒定能量轮廓构成连接到三重简并结点的表面投影的“费米弧”对，证明了新识别的半金属状态的非平凡拓扑。