  The standard electroweak theory admits a string solution, the Z string, in
which only the electrically neutral Higgs fields are excited. This solution is
unstable at zero temperature: Z strings decay by exciting charged Higgs modes.
In the early Universe, however, there was a long period during which the Higgs
particles were out of equilibrium but the photon field was in thermal
equilibrium. We show that in this phase Z strings are stabilized by
interactions of the charged Higgs modes with the photons. In a first
temperature range immediately below the electroweak symmetry breaking scale,
the stabilized embedded defects are symmetric in internal space (the charged
scalar fields are not excited). There is a second critical temperature below
which the stabilized embedded strings undergo a core phase transition and the
charged scalar fields take on a nonvanishing value in the core of the strings.
We show that stabilized embedded defects with an asymmetric core persist to
very low temperatures. The stabilization mechanism discussed in this paper is a
prototypical example of a process which will apply to a wider class of embedded
defects in gauge theories.
