  We study the scattering of fermions off the true vacuum bubbles nucleated
during a first order electroweak phase transition in the presence of a
background hypermagnetic field. By considering the limit of an infinitely thin
bubble wall, we derive and analytically solve the Dirac equation in three
spatial dimensions for left and right chirality modes and compute reflection
and transmission coefficients for the case when the fermions move from the
symmetric toward the broken symmetry phase. Given the chiral nature of the
fermion coupling with the background field in the symmetric phase, an axial
asymmetry is generated during the scattering processes. This mechanism is thus
shown to provide an additional source of CP violation within the standard
model. We discuss possible implications for baryon number generation.
