The electroweak phase transition in a magnetic field Original Research Article

We study the finite temperature electroweak phase transition in an external hypercharge U(1) magnetic field HY, using lattice Monte Carlo simulations. For sufficiently small fields, HY/T2⪅ 0.3, the magnetic field makes the first-order transition stronger, but it still turns into a crossover for Higgs masses mH ⪅ 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 HY, penetrates through the broken phase. For the magnetic fields and Higgs masses studied, we did not see indications of the expected Ambjørn-Olesen phase, which should be similar to a type II superconductor.