Stabilization of the Electroweak Z String in the Early Universe

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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.