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

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 $γ$, $Z$, 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 $ΔL=0$ 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.

Figures (23)

• Figure 1: Representative one-loop matching diagrams generating the gauge-invariant electromagnetic dipole operators $\mathcal{O}_{NB}$ and $\mathcal{O}_{NW}$. These diagrams make explicit the EFT origin of the interactions that enter the Kadanoff--Baym equations.
• Figure 2: Feynman diagrams for the two-body decays of $N_i$ induced by the dipole operators $\mathcal{O}_{NB},\mathcal{O}_{NW}$. The gray blob indicates that each effective vertex incorporates the UV information of the heavy fields $E,\Sigma,D,\Phi^+$ through the Wilson coefficients $C_{NB}$ and $C_{NW}$.
• Figure 3: Two vertex-type one-loop diagrams contributing to the photon-mode CP asymmetry. The diagrams (a) and (b) differ by the fermion-number orientation of the internal light-neutrino line $\nu_\beta(q_1)$, which determines whether the dipole vertex on that line is $V$ or its Hermitian-conjugate form $\tilde{V}$.
• Figure 4: Cutkosky cut for the vertex contribution. The vertical dashed line indicates the cut, which places the internal photon line and the internal light-neutrino line on shell.
• Figure 5: Self-energy-type one-loop diagrams contributing to the photon mode.