  Simply based on CP arguments, we argue against a Standard Model explanation
of the baryon asymmetry of the universe in the presence of a first order phase
transition. A CP-asymmetry is found in the reflection coefficients of quarks
hitting the phase boundary created during the electroweak transition. The
problem is analyzed both in an academic zero temperature case and in the
realistic finite temperature one. The building blocks are similar in both
cases: Kobayashi-Maskawa CP-violation, CP-even phases in the reflection
coefficients of quarks, and physical transitions due to fermion self-energies.
In both cases an effect is present at order $\alpha_W^2$ in rate. A standard
GIM behaviour is found as intuitively expected. In the finite temperature case,
a crucial role is played by the damping rate of quasi-particles in a hot
plasma, which is a relevant scale together with $M_W$ and the temperature. The
effect is many orders of magnitude below what observation requires, and
indicates that non standard physics is indeed needed in the cosmological
scenario.
