  Reduction in effective spacetime dimensionality can occur in field-theory
models more general than the widely studied dimensional reductions based on
technically consistent truncations. Situations where wavefunction factors
depend nontrivially on coordinates transverse to the effective lower dimension
can give rise to unusual patterns of gauge symmetry breaking. Leading-order
gauge modes can be left massless, but naturally occurring Stueckelberg modes
can couple importantly at quartic order and higher, thus generating a "covert"
pattern of gauge symmetry breaking. Such a situation is illustrated in a
five-dimensional model of scalar electrodynamics in which one spatial dimension
is taken to be an interval with Dirichlet/Robin boundary conditions on opposing
ends. This simple model illuminates a mechanism which also has been found in
gravitational braneworld scenarios.
