Thermal Fermion Propagators and Flavor-Changing via Loop Corrections in the Early Universe

We present the Minimally Extended Standard Model in the mass basis in the symmetric phase of the early Universe, demonstrating that the CP-violating CKM and PMNS matrices emerge not only in the weak interactions but also in the Yukawa interactions. This is due to the presence of all four components of the Higgs field. Specifically, left-handed fermions acquire additional thermal mass contributions involving the CKM and PMNS matrix elements. Most notably, the structure of Yukawa interactions enables flavor-changing and CP-violating conversions among left-handed fermions, such as $u_L\leftrightarrow c_L$, which do not occur in the broken phase. In contrast, the right-handed fermions retain their mass eigenstates, leading to distinct thermal masses. Additionally, we identify novel CP-violating scattering processes, such as $u_L\bar{c}_L\rightarrow 2A_3$, which are exclusive to the symmetric phase.