  Based on a detailed analysis of nonlinear field equations of the SU(2)
Yang-Mills-Higgs system, we obtain the effective field theory describing
low-energy interaction of BPS dyons and massless particles (i.e., photons and
Higgs particles). Our effective theory manifests electromagnetic duality and
spontaneously broken scale symmetry, and reproduces the multimonopole moduli
space dynamics of Manton in a suitable limit. Also given is a generalization of
our approach to the case of BPS dyons in a gauge theory with an arbitrary gauge
group that is maximally broken.
