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The spin fluctuations parallel to the external magnetic field in the ground state of the one-dimensional (1D) s=1/2 Heisenberg antiferromagnet are dominated by a two-parameter set of collective excitations. In a cyclic chain of N sites and magnetization 0 <Mz < N/2, the ground state, which contains 2Mz spinons, is reconfigured as the physical vacuum for a different species of quasiparticles, identifiable in the framework of the coordinate Bethe ansatz by characteristic configurations of Bethe quantum numbers. The dynamically dominant excitations are found to be scattering states of two such quasiparticles. For N --> ∞, these collective excitations form a continuum in (q, ω) space with an incommensurate soft mode. Their matrix elements in the dynamic spin structure factor Szz (q, ω) are calculated directly from the Bethe wave functions for finite N. The resulting line-shape predictions for N --> ∞ complement the exact results previously derived via algebraic analysis for the exact two-spinon part of Szz (q, ω) in the zero-field limit. They are relevant for neutron-scattering experiments on quasi-1D antiferromagnetic compounds in a strong magnetic field.

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© 2000 The American Physical Society