  Low-energy excitations in spin 1/2 antiferromagnetic Heisenberg spin ladders
are studied by bosonization and gauge theoretical description. It is explicitly
shown that zero modes in the bosonization play an essentially important role.
Low-energy excitations in the 2-leg system is described by spin triplet
Majorana fermions, and in the 3-leg case spin excitations on the top and bottom
ladders are described by two massless scalar fields. It is predicted that if
inter-ladder interaction between the top and bottom chains is introduced, a
phase transition occures at some critical coupling and the system acquires an
energy gap.
