New Constraint on Primordial Lepton Flavor Asymmetries

A chiral chemical potential present in the early Universe can source helical hypermagnetic fields through the chiral plasma instability. If these hypermagnetic fields survive until the electroweak phase transition, they source a contribution to the baryon asymmetry of the Universe. In this Letter, we demonstrate that lepton flavor asymmetries above |μ |/T ∼9 ×10^-3 trigger this mechanism even for vanishing total lepton number. This excludes the possibility of such large lepton flavor asymmetries present at temperatures above 10⁶ GeV , setting a constraint which is about 2 orders of magnitude stronger than the current CMB and BBN limits.