  In a Dirac semimetal, the massless Dirac fermion has zero chirality, leading
to surface states connected adiabatically to a topologically trivial surface
state as well as vanishing anomalous Hall effect (AHE). Recently, it is
predicted that in the nonrelativistic limit of certain collinear
antiferromagnets, there exists a type of chiral Dirac-like fermion, whose
dispersion manifests four-fold degenerate crossing points formed by
spin-degenerate linear bands, with topologically protected Fermi arcs. Such
unconventional chiral fermion, protected by a hidden SU(2) symmetry in the
hierarchy of an enhanced crystallographic group, namely spin space group, is
not experimentally verified yet. Here, by angle-resolved photoemission
spectroscopy measurements, we reveal the surface origin of the electron pocket
at the Fermi surface in collinear antiferromagnet CoNb3S6. Combining with
neutron diffraction and first-principles calculations, we suggest a multidomain
collinear AFM configuration, rendering the the existence of the Fermi-arc
surface states induced by chiral Dirac-like fermions. Our work provides
spectral evidence of the chiral Dirac-like fermion caused by particular spin
symmetry in CoNb3S6, paving an avenue for exploring new emergent phenomena in
antiferromagnets with unconventional quasiparticle excitations.
