  Uncovering the physical contents of the nontrivial topology of quantum states
is a critical problem in condensed matter physics. Here, we study the
topological circular dichroism in chiral semimetals using linear response
theory and first-principles calculations. We show that, when the low-energy
spectrum respects emergent SO(3) rotational symmetry, topological circular
dichroism is forbidden for Weyl fermions, and thus is unique to chiral
multifold fermions. This is a result of the selection rule that is imposed by
the emergent symmetry under the combination of particle-hole conjugation and
spatial inversion. Using first-principles calculations, we predict that
topological circular dichroism occurs in CoSi for photon energy below about 0.2
eV. Our work demonstrates the existence of a response property of
unconventional fermions that is fundamentally different from the response of
Dirac and Weyl fermions, motivating further study to uncover other unique
responses.
