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 I & mu;I=T & SIM; 9 x 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.

Automated summary

This letter investigates the relationship between helical hypermagnetic fields and fermion asymmetry in the early Universe caused by chiral chemical potential. The study demonstrates that even with zero total lepton number, a mechanism can be triggered through lepton flavor asymmetry. This sets a constraint on lepton flavor asymmetry at temperatures above 106 GeV, which is about two orders of magnitude stronger than current limits from cosmic microwave background and primordial nucleosynthesis.