  In a recent neutron-scattering experiment on the quasi-one-dimensional
$S=1/2$ antiferromagnet Cu Benzoate, a gap was induced by an applied magnetic
field. We argue that the primary mechanism of the gap formation is an effective
staggered field due to both the alternating $g$-tensor and the
Dzyaloshinskii-Moriya interaction. We explain the dependence of the gap on the
applied field, as well as identify several peaks in the structure factor
$S(q,\omega)$.
