We use a quantum O(N) nonlinear sigma model to describe one-dimensiona
l quantum Heisenberg antiferromagnets with integer spin S. A large N a
nalytical treatment of this effective theory leads to definite predict
ions for thermodynamic quantities in the low-temperature regime. We ob
tain the Haldane gap as a function of the temperature and note a trend
similar to that found in the compound NENP. The specific heat and mag
netic susceptibility are also obtained and comparison with exact numer
ical results for the S = 1 spin chain allows us to determine the domai
n of validity of our approximation. By a calculation at subleading 1/N
order we compute the quasiparticle lifetime. Results are given on-she
ll, for the width observed at the quasiparticle pole as seen in neutro
n scattering experiments, and also off-shell, for the NMR relaxation r
ate T1(-1). The phenomenological interacting boson model is compared w
ith our approach. We discuss the effects of anisotropy and compare our
results with present experiments on NENP.