Fermion Density Induced Instability of the W-Boson Pair Condensate in Strong Magnetic Field

The electroweak vacuum structure in an external magnetic field close to the lower critical value is considered at finite fermion density. It is shown that the leading effect of the fermions is to reduce the symmetry of the W-pair condensate in the direction of the magnetic field. The energy is minimized by the appearance of a helicoidal structure of the condensate along the magnetic field.