  In the O(3) sigma-model description of gapped spin systems, S=1 magnons can
only decay into three lower energy magnons. We argue that the symmetry of the
quantum spin Hamiltonian often allows decay into two magnons, and compute this
decay rate in model systems. Two magnon decay is present in Haldane gap S=1
spin chains, even though it cannot be induced by any allowed term written in
powers and gradients of the sigma-model field. We compare our results with
recent measurements of Stone et al. (cond-mat/0511266) on a two-dimensional
spin system.
