  We have performed angle-resolved photoemission spectroscopy on HfSiS, which
has been predicted to be a topological line-node semimetal with square Si
lattice. We found a quasi-two-dimensional Fermi surface hosting bulk nodal
lines, alongside the surface states at the Brillouin-zone corner exhibiting a
sizable Rashba splitting and band-mass renormalization due to many-body
interactions. Most notably, we discovered an unexpected Dirac-like dispersion
extending one-dimensionally in k space - the Dirac-node arc - near the bulk
node at the zone diagonal. These novel Dirac states reside on the surface and
could be related to hybridizations of bulk states, but currently we have no
explanation for its origin. This discovery poses an intriguing challenge to the
theoretical understanding of topological line-node semimetals.
