Cu benzoate in an applied magnetic field can be described by a S = 1/2 Heis
enberg antiferromagnetic chain with an effective transverse staggered field
. The low-energy physics of such a system can be described by the quantum s
ine-Gordon field theory. ESR measurements of Cu benzoate at very low temper
ature probes the non-perturbative dynamics of the sine-Gordon field theory.