  As first demonstrated by Tang and Cohen in chiral optics, the asymmetry in
the rate of electromagnetic energy absorption between left and right
enantiomers is determined by an optical chirality density [1]. Here, we
demonstrate that this effect can exist in magnetic spin systems. By
constructing a formal analogy with electrodynamics, we show that in
antiferromagnets with broken chiral symmetry the asymmetry in local spin-wave
energy absorption is proportional to a spin-wave chirality density, which is a
direct counterpart of optical zilch. We propose that injection of a pure spin
current into an antiferromagnet may serve as a chiral symmetry breaking
mechanism, since its effect in the spin-wave approximation can be expressed in
terms of additional Lifshitz invariants. We use linear response theory to show
that the spin current induces a nonequilibrium spin-wave chirality density.
