  We calculate the splitting between fermion and anti-fermion effective masses
in high temperature gauge theories in the presence of a non-vanishing chemical
potential due to the $CP$-asymmetric fermionic background. In particular we
consider the case of left-handed leptons in the $SU(2)\otimes U(1)$ theory when
the temperature is above $250$ GeV and the gauge symmetry is restored.

High Energy Physics - Phenomenology