  We report the optical conductivity in high-quality crystals of the chiral
topological semimetal CoSi, which hosts exotic quasiparticles known as
multifold fermions. We find that the optical response is separated into several
distinct regions as a function of frequency, each dominated by different types
of quasiparticles. The low-frequency intraband response is captured by a narrow
Drude peak from a high-mobility electron pocket of double Weyl quasi-particles,
and the temperature dependence of the spectral weight is consistent with its
Fermi velocity. By subtracting the low-frequency sharp Drude and phonon peaks
at low temperatures, we reveal two intermediate quasi-linear inter-band
contributions separated by a kink at 0.2 eV. Using Wannier tight-binding models
based on first-principle calculations, we link the optical conductivity above
and below 0.2 eV to interband transitions near the double Weyl fermion and a
threefold fermion, respectively. We analyze and determine the chemical
potential relative to the energy of the threefold fermion, revealing the
importance of transitions between a linearly dispersing band and a flat band.
More strikingly, below 0.1 eV our data are best explained if spin-orbit
coupling is included, suggesting that at these energies the optical response is
governed by transitions between a previously unobserved four-fold spin-3/2 node
and a Weyl node. Our comprehensive combined experimental and theoretical study
provides a way to resolve different types of multifold fermions in CoSi at
different energy. More broadly our results provide the necessary basis to
interpret the burgeoning set of optical and transport experiments in chiral
topological semimetals.
