Lepton asymmetry and the cosmic QCD transition

We study the influence of lepton asymmetries on the evolution of the early Universe. The lepton asymmetry l is poorly constrained by observations and might be orders of magnitudes larger than the observed baryon asymmetry b ≃ 10 −10 , | l |/ b ⩽ 2 × 10 8 . We find that lepton asymmetries that are large compared to the tiny baryon asymmetry, can influence the dynamics of the QCD phase transition significantly. The cosmic trajectory in the μ B − T phase diagram of strongly interacting matter becomes a function of lepton (flavour) asymmetry. For tiny or vanishing baryon and lepton asymmetries lattice QCD simulations show that the cosmic QCD transition is a rapid crossover. However, for large lepton asymmetry, the order of the cosmic transition remains unknown.