Bound States Can Stabilize Electroweak Strings
  We show that the electroweak $Z-$string can be stabilized by the presence of bound states of a complex scalar field. We argue that fermions coupled to the scalar field of the string can also make the string stable and discuss the physical case where the string is coupled to quarks and leptons. This stabilization mechanism is expected to work for other embedded defects and also for unstable solutions such as the sphaleron. 
Tanmay Vachaspati and Richard Watkins