Could a hypothetical gauge field help explain why matter survived?

A cosmic imbalance with a possible field-based origin

Nearly everything we see is made of matter rather than antimatter. To explain this imbalance, physicists investigate processes in the early Universe that could have generated a small excess of matter. Kohei Kamada, Fumio Uchida and Di Wu study one proposed route: a hypothetical gauge field produced during inflation might eventually help create the imbalance.

The paper focuses on a field associated with a symmetry called U(1)_{Lμ−Lτ}, which distinguishes muon- and tau-related lepton numbers. This is not the ordinary electromagnetic field or an observed astrophysical magnetic field. The authors consider an axion-like inflaton—a field driving inflation—that couples to the hypothetical gauge field. In their model, the coupling amplifies one circular polarization, producing magnetic helicity: a measure of how twisted the field is.

The central question is what happens after inflation. As the Universe reheats into a hot plasma, the field can lose helicity. Through the chiral anomaly, that change can generate a lepton imbalance. Electroweak sphaleron processes can then convert part of that imbalance into a baryon imbalance. Baryons include protons and neutrons, the building blocks of ordinary matter.

Following the field and particle balances

The researchers model the field’s post-reheating evolution using magnetohydrodynamics, the framework for magnetic fields in conducting fluids. They consider two broad cases. If fluid motion strongly affects the field, it may pass through an initially nearly frozen phase and later turbulent evolution. If fluid motion is less important, electrical resistance, or Ohmic diffusion, can dominate the field’s decay.

They connect the changing helicity to equations tracking particle asymmetries in the hot plasma. These Boltzmann equations include lepton-number production, interactions that redistribute particle imbalances, and sphaleron processes. The authors compare numerical solutions with a simpler chemical-equilibrium approximation and examine which combinations of the inflationary expansion rate and initial helicity could reproduce the observed baryon asymmetry.

Their calculations identify parameter regions where the model can yield the observed asymmetry in both turbulent and diffusion-dominated cases. But “turbulent” does not mean turbulence does most of the work: in the turbulent case, the calculated baryon production is mainly set during an earlier, nearly frozen phase, before fully developed turbulence. The outcome therefore depends importantly on the field’s early post-reheating evolution.

The authors also find that requiring successful baryogenesis can impose tighter limits on the inflationary Hubble parameter and initial helicity than requiring only that the gauge-field energy remain too small to disrupt inflation. Their parameter-space analysis also considers excessive baryon production and limits related to a later lepton asymmetry’s effect on primordial helium. These are conditional results within the model, not evidence that the mechanism operated in the Universe.

Assumptions and open questions

A central assumption is that Schwinger pair production—creation of charged particle pairs in a strong electric field—is suppressed during inflation. This allows the model to generate substantial helicity while beginning with negligible lepton asymmetry. If the charged particles remain light, their production can affect both the field and particle asymmetries; the authors say that case requires a self-consistent treatment of their coupled evolution.

The later disappearance of the helicity is also uncertain. The calculations adopt a particular account of magnetic-field evolution and of what happens when the gauge symmetry breaks. The paper notes that more complicated processes, potentially involving cosmic strings, could change the timing or efficiency of helicity decay and lepton production. For the turbulent case, the authors use a published scaling classification, while also noting that numerical investigations of this topic have not been well explored. The classification should therefore not be taken as a direct numerical validation of this specific scenario.

This is a theoretical analysis, not an observation of the proposed field or a measurement of the Universe’s original particle imbalance. The supplied paper version is an arXiv preprint prepared for journal submission; its conclusions should be read as dependent on its assumptions and modelling choices, not as settled evidence. It develops a possible mechanism and identifies constraints, while leaving important questions about inflation-era particle production and later field decay open.