  In the broken-symmetry phase of the electroweak theory there is no unique
definition of the electromagnetic field tensor in cases where the magnitude of
the Higgs field differs from a constant value. The meaning of the
electromagnetic field is therefore dubious near defects and during
non-equilibrium stages of the electroweak phase transition. Nevertheless, by
imposing a minimal set of natural requirements one is led to a specific,
gauge-invariant definition that retains the familiar properties of an
electromagnetic field. An electromagnetic vector potential is constructed whose
curl (exterior derivative) in any gauge gives the electromagnetic field tensor.
As is required, this vector potential transforms at most by a pure gradient
under arbitrary SU(2)xU(1) gauge transformations. The flux of the magnetic
field is expressed as a gauge-invariant line integral. Curiously, this provides
a definition for magnetic flux in cases where the spatial region with broken
symmetry is not simply connected and the magnetic field itself is not
everywhere defined.
