We study the finite temperature electroweak phase transition in an external hypercharge U(1) magnetic field H_Y, using lattice Monte Carlo simulations. For sufficiently small fields, H_Y/T^2 < 0.3, the magnetic field makes the first order transition stronger, but it still turns into a crossover for Higgs masses m_H ~ 80 GeV. For larger fields, we observe a mixed phase analogous to a type I superconductor, where a single macroscopic tube of the symmetric phase, parallel to H_Y, penetrates through the broken phase. For the magnetic fields and Higgs masses studied, we did not see indications of the expected Ambjorn-Olesen phase, which should be similar to a type II superconductor.
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