  We report inelastic time-of-flight and triple-axis neutron scattering
measurements of the excitation spectrum of the coupled antiferromagnetic spin-1
Heisenberg chain system CsNiCl3. Measurements over a wide range of wave-vector
transfers along the chain confirm that above T_N CsNiCl3 is in a
quantum-disordered phase with an energy gap in the excitation spectrum. The
spin correlations fall off exponentially with increasing distance with a
correlation length xi=4.0(2) sites at T=6.2K. This is shorter than the
correlation length for an antiferromagnetic spin-1 Heisenberg chain at this
temperature, suggesting that the correlations perpendicular to the chain
direction and associated with the interchain coupling lower the single-chain
correlation length. A multi-particle continuum is observed in the
quantum-disordered phase in the region in reciprocal space where
antiferromagnetic fluctuations are strongest, extending in energy up to twice
the maximum of the dispersion of the well-defined triplet excitations. We show
that the continuum satisfies the Hohenberg-Brinkman sum rule. The dependence of
the multi-particle continuum on the chain wave-vector resembles that of the
two-spinon continuum in antiferromagnetic spin-1/2 Heisenberg chains. This
suggests the presence of spin-1/2 degrees of freedom in CsNiCl3 for T < 12K,
possibly caused by multiply-frustrated interchain interactions.
