  Multifold fermions are generalizations of two-fold degenerate Weyl fermions
with three-, four-, six- or eight-fold degeneracies protected by crystal
symmetries, of which only the last type is necessarily non-chiral. Their low
energy degrees of freedom can be described as emergent particles not present in
the Standard Model of particle physics. We propose a range of experimental
probes for multifold fermions in chiral symmetry groups based on the gyrotropic
magnetic effect (GME) and the circular photo-galvanic effect (CPGE). We find
that, in contrast to Weyl fermions, multifold fermions can have zero Berry
curvature yet a finite GME, leading to an enhanced response. The CPGE is
quantized and independent of frequency provided that the frequency region at
which it is probed defines closed optically-activated momentum surfaces. We
confirm the above properties by calculations in symmetry-restricted tight
binding models with realistic density functional theory parameters. We identify
a range of previously-unidentified ternary compounds able to exhibit chiral
multifold fermions of all types (including a range of materials in the families
AsBaPt and Gd$_3$Cl$_3$C), and provide specific predictions for the known
multifold material RhSi.
