We show that the $Z$ strings of the standard electroweak theory can be stabilized by strong external magnetic fields, provided that ${\beta }^{1/2}\equiv M_H/M_Z\le 1$, where $M_H$ and $M_Z$ are the Higgs and $Z$ masses. The magnetic fields needed are larger than ${\beta }^{1/2}{B}_c$ and smaller than $B_c$, where $B_c\equiv M_W^2/e$ is the critical magnetic field which causes $W$ condensation in the usual broken phase vacuum. If such magnetic fields were present after the electroweak transition, they would stabilize strings for a period comparable to the inverse Hubble rate at that time. Pair creation of monopoles and antimonopoles linked by segments of string is briefly considered.

• Received 30 May 1995

DOI:https://doi.org/10.1103/PhysRevLett.75.2268