Baryogenesis from Decaying Magnetic Helicity

As a result of the Standard Model chiral anomalies, baryon number is violated in the early universe in the presence of a hypermagnetic field with varying helicity. We investigate whether the matter / anti-matter asymmetry of the universe can be created from the decaying helicity of a primordial (hyper)magnetic field before and after the electroweak phase transition. In this model, baryogenesis occurs without $(B-L)$-violation, since the $(B+L)$ asymmetry generated by the hypermagnetic field counteracts the washout by electroweak sphalerons. At the electroweak crossover, the hypermagnetic field becomes an electromagnetic field, which does not source $(B+L)$. Although the sphalerons remain in equilibrium for a time, washout is avoided since the decaying magnetic helicity sources chirality. The relic baryon asymmetry is fixed when the electroweak sphaleron freezes out. Under reasonable assumptions, a baryon asymmetry of $n_B / s \simeq 4 \times 10^{-12}$ can be generated from a maximally helical, right-handed (hyper)magnetic field that has a field strength of $B_0 \simeq 10^{-14} \, {\rm Gauss}$ and coherence length of $\lambda_{0} \simeq 1 \, {\rm pc}$ today. Relaxing an assumption that relates $\lambda_0$ to $B_0$, the model predicts $n_B / s \gtrsim 10^{-10}$, which could potentially explain the observed baryon asymmetry of the universe.