  The T=0 dynamical properties of the one-dimensional (1D) $s=1/2$
  Heisenberg antiferromagnet in a uniform magnetic field are studied via Bethe
ansatz for cyclic chains of $N$ sites. The ground state at magnetization
$0<M_z<N/2$, which can be interpreted as a state with $2M_z$ spinons or as a
state of $M_z$ magnons, is reconfigured here as the vacuum for a different
species of quasiparticles, the {\em psinons} and {\em antipsinons}. We
investigate three kinds of quantum fluctuations, namely the spin fluctuations
parallel and perpendicular to the direction of the applied magnetic field and
the dimer fluctuations. The dynamically dominant excitation spectra are found
to be sets of collective excitations composed of two quasiparticles excited
from the psinon vacuum in different configurations. The Bethe ansatz provides a
framework for (i) the characterization of the new quasiparticles in relation to
the more familiar spinons and magnons, (ii) the calculation of spectral
boundaries and densities of states for each continuum, (iii) the calculation of
transition rates between the ground state and the dynamically dominant
collective excitations, (iv) the prediction of lineshapes for dynamic structure
factors relevant for experiments performed on a variety of quasi-1D
antiferromagnetic compounds, including KCuF$_3$,
  Cu(C$_4$H$_4$N$_2)(NO_3)_2$, and CuGeO$_3$.
