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 <math display="inline"><mo stretchy="false">|</mo><mi>μ</mi><mo stretchy="false">|</mo><mo>/</mo><mi>T</mi><mo>∼</mo><mn>9</mn><mo>×</mo><msup><mn>10</mn><mrow><mo>-</mo><mn>3</mn></mrow></msup></math> trigger this mechanism even for vanishing total lepton number. This excludes the possibility of such large lepton flavor asymmetries present at temperatures above <math display="inline"><mrow><msup><mrow><mn>10</mn></mrow><mrow><mn>6</mn></mrow></msup><mtext> </mtext><mtext> </mtext><mi>GeV</mi></mrow></math>, setting a constraint which is about 2 orders of magnitude stronger than the current CMB and BBN limits.