Protecting the Baryon Asymmetry with Thermal Masses

We consider the evolution of baryon number $B$ in the early universe under the influence of rapid sphaleron interactions and show that $B$ will remain nonzero at all times even in the case of $B-L=0$. This result arises due to thermal Yukawa interactions that cause nonidentical dispersion relations (thermal masses) for different lepton families. We point out the relevance of our result to the Affleck-Dine type baryogenesis.