  Plasmonics in topological semimetals offers exciting opportunities for
fundamental physics exploration as well as for technological applications.
Here, we investigate plasmons in the exemplar chiral crystal CoSi, which hosts
a variety of multifold fermionic excitations. We show that CoSi hosts two
distinct plasmon modes in the infrared regime at 0.1 eV and 1.1 eV in the
long-wavelength limit. The 0.1 eV plasmon is found to be highly dispersive, and
originates from intraband collective oscillations associated with a double
spin-1 excitation, while the 1.1 eV plasmon is dispersionless and it involves
interband correlations. Both plasmon modes lie outside the particle-hole
continuum and possess long lifetime. Our study indicates that the CoSi class of
materials will provide an interesting materials platform for exploring
fundamental and technological aspects of topological plasmonics.
