  In 1929, H. Weyl proposed that the massless solution of Dirac equation
represents a pair of new type particles, the so-called Weyl fermions [1].
However the existence of them in particle physics remains elusive for more than
eight decades. Recently, significant advances in both topological insulators
and topological semimetals have provided an alternative way to realize Weyl
fermions in condensed matter as an emergent phenomenon: when two non-degenerate
bands in the three-dimensional momentum space cross in the vicinity of Fermi
energy (called as Weyl nodes), the low energy excitation behaves exactly the
same as Weyl fermions. Here, by performing soft x-ray angle-resolved
photoemission spectroscopy measurements which mainly probe bulk band structure,
we directly observe the long-sought-after Weyl nodes for the first time in
TaAs, whose projected locations on the (001) surface match well to the Fermi
arcs, providing undisputable experimental evidence of existence of Weyl fermion
quasiparticles in TaAs.
