Feshbach resonances and dynamics of BPS solitons

We demonstrate that the geodesic dynamics of Bogomol’nyi–Prasad–Sommerfield solitons can be modified by the excitation of Feshbach resonances, or quasibound modes, in a toy model of two scalar fields. A mode-generated force emerges, with a strength determined by the spectral flow of the frequency on the moduli space, and weakened by the coupling between the bound and scattering components of the resonance. Notably, spectral walls persist unaffected by the resonant mode’s exponential decay, as the decay constant vanishes at the spectral wall. Our motivation comes from the ’t Hooft-Polyakov monopoles, which do not present true bound states but long lived semibound excitations. Our findings suggest the existence of spectral walls in the scattering of excited monopoles in three dimensions, whose trajectories may significantly deviate from the geodesic motion in the moduli space of unexcited monopoles. Published by the American Physical Society 2025