Detailed Stability Analysis of Electroweak Strings
  We give a detailed stability analysis of the Z-string in the standard electroweak model. We identify the mode that determines the stability of the string and numerically map the region of parameter space where the string is stable. For $\sin^2 \theta_W = 0.23$, we find that the strings are unstable for a Higgs mass larger than 23GeV. Given the latest constraints on the Higgs mass from LEP, this shows that, if the standard electroweak model is realized in Nature, the existing vortex solutions are unstable. 
Margaret James, Leandros Perivolaropoulos and Tanmay Vachaspati