New Constraint on Primordial Lepton Flavor Asymmetries

Area of Science:

• Cosmology
• Particle Physics
• Astrophysics

Background:

• Chiral chemical potentials in the early Universe can generate helical hypermagnetic fields via the chiral plasma instability.
• These hypermagnetic fields, if surviving until the electroweak phase transition, contribute to the Universe's baryon asymmetry.

Purpose of the Study:

• To investigate the role of lepton flavor asymmetries in sourcing hypermagnetic fields and baryon asymmetry.
• To establish new constraints on early Universe conditions using lepton flavor asymmetries.

Main Methods:

• Theoretical analysis of the chiral plasma instability mechanism.
• Investigating the conditions under which lepton flavor asymmetries trigger hypermagnetic field generation.
• Comparing derived constraints with existing limits from Cosmic Microwave Background (CMB) and Big Bang Nucleosynthesis (BBN).

Main Results:

• Lepton flavor asymmetries exceeding |μ|/T∼9×10^{-3} can trigger hypermagnetic field generation even with zero total lepton number.
• This mechanism provides a new pathway to generate baryon asymmetry in the early Universe.

Conclusions:

• The study excludes the possibility of large lepton flavor asymmetries at temperatures above 10^6 GeV.
• This research sets a new, stringent constraint on early Universe physics, approximately two orders of magnitude stronger than current CMB and BBN limits.