Schwinger--Keldysh formulation of electromagnetic leptogenesis in an EFT framework

TL;DR

This paper develops a quantum kinetic framework using Schwinger--Keldysh formalism to analyze electromagnetic leptogenesis at TeV scales within an EFT, highlighting resonant effects and flavor coherence for baryon asymmetry generation.

Contribution

It introduces a density-matrix quantum kinetic equation approach for electromagnetic leptogenesis, incorporating resonant CP asymmetry and flavor coherence effects in an EFT context.

Findings

Resonant enhancement of CP asymmetry in quasi-degenerate neutrinos.

Numerical solutions show potential overproduction of baryon asymmetry.

Unified treatment of scattering processes without RIS subtraction.

Abstract

We study TeV-scale electromagnetic leptogenesis (EMLG) in an effective field theory (EFT) framework, starting from an ultraviolet (UV)-complete model in which integrating out heavy states generates the gauge-invariant dimension-six dipole operators $O_{NB}$ and $O_{NW}$. After electroweak symmetry breaking these operators induce effective dipole couplings of the right-handed neutrinos $N$ to $\gamma$, $Z$, and $W^{\pm}$, enabling decays, inverse decays, and scattering processes in the electroweak crossover window. Gauge invariance enforces a Higgs insertion and thus a parametric suppression of both the CP-odd source and the washout in the non-resonant, hierarchical regime, preventing reproduction of the observed baryon asymmetry. We therefore focus on the quasi-degenerate limit, in which the self-energy contribution to the CP asymmetry is resonantly enhanced and coherent flavor dynamics among the nearly degenerate $N$ becomes essential. Using the CTP/Schwinger--Keldysh formalism, we derive density-matrix quantum kinetic equations (QKEs) whose collision term, at leading order in the effective dipole couplings and SM gauge interactions, incorporates $1\leftrightarrow 2$ processes and the leading $\Delta L=0$ scatterings in a unified and non-overcounted manner, without the need for real-intermediate-state (RIS) subtraction. Solving the resulting system numerically, we present a coherent EFT pipeline from UV matching and renormalization-group (RG) running to electroweak-scale effective dipole couplings, resonantly regulated CP sources, and the frozen-out baryon asymmetry. For both thermal and zero initial heavy-neutrino abundances, we find in the oscillation-motivated region that the undiluted freeze-out yield $Y_B^{\rm FO}$ can exceed $Y_B^{\rm obs}\simeq 8.7\times10^{-11}$ by several orders of magnitude.