  The Goldstone mode in a helical magnetic phase, also known as the helimagnon,
is a propagating mode with a highly anisotropic dispersion relation. Here we
study theoretically the helimagnon excitations in a complex chiral ground state
of pyrochlore antiferromagnets such as spinel CdCr2O4 and itinerant magnet
YMn2. We show that the effective theory of the soft modes in the helical state
possesses a symmetry similar to that of smectic liquid crystals. We compute the
low-energy spin-wave spectrum based on a microscopic spin Hamiltonian and find
a dispersion relation characteristic of the helimagnons. By performing dynamics
simulations with realistic model parameters, we also obtain an overall
agreement between experiment and the numerical spin-wave spectrum. Our work
thus also clarifies the mechanisms that relive the magnetic frustration in
these compounds.
