  The effect of thermal fluctuations on the dynamics of a gapped quantum magnet
is studied using inelastic neutron scattering on copper nitrate, a model
material for the one-dimensional (1D) bond alternating Heisenberg chain,
combined with theoretical and numerical analysis. We observe and interpret the
thermally induced central peak due to intraband scattering as well as the
thermal development of an asymmetric continuum of scattering. We relate this
asymmetric line broadening to hard core constraints and quasi-particle
interactions. Our findings are a counter example to recent assertions of
universality of line broadening in 1D systems and are to be considered as a new
paradigm of behaviour, applicable to a broad range of quantum systems.
