Emergent Large Lepton Mixing from Neutrino Refraction in Dark Matter

Abstract

We propose a novel origin for the disparity between quark and lepton flavor mixing based on the refractive nature of neutrino masses. We postulate that the fundamental mixing in both the quark and lepton sectors is CKM-like, together with tiny vacuum neutrino masses, while the observed PMNS mixing matrix emerges dynamically from coherent forward scattering of neutrinos on an ultralight dark matter background. The resulting in-medium Hamiltonian rotates CKM mixing angles into large effective lepton mixings, naturally realizing quark--lepton complementarity without invoking new flavor symmetries. This framework links neutrino mass generation, flavor mixing, and dark matter, and predicts environment-dependent neutrino oscillation effects testable in current and future experiments.

Figures (5)

• Figure 1: Illustration of our framework. In vacuum (left), neutrino mixing is CKM-like represented by the flavor content of the true mass eigenstates $\tilde{\nu}_i$. Meanwhile, in a uniform dark background experiments would measure a PMNS-like mixing summarized in the flavor composition of eigenstates in the dark-matter $\nu_i$.
• Figure 2: Solar neutrino flux-averaged survival probability in the Standard scenario (black) and in our model for $\delta m = 10^{-4}~{\rm eV}$ (purple), $10^{-3}~{\rm eV}$ (dashed blue), $2\times 10^{-3}~{\rm eV}$ (dotted orange), and $5\times 10^{-3}~{\rm eV}$ (dot-dashed light blue). The gray data indicate the different measurements from solar neutrino experiments.
• Figure 3: Atmospheric neutrino oscillogram for the $\nu_e\to\nu_\mu$ channel in terms of neutrino energy and zenith angle. We present the standard case (left) together with the oscillation pattern in our framework with $\delta m=10^{-2}~{\rm eV}$ (right).
• Figure 4: $\nu_e$ appearance probability $P(\nu_\mu\to\nu_e)$ as function of neutrino energy for different long baseline experiments, T2K T2K:2023smv (left), NO$\nu$A NOvA:2021nfi (middle) and DUNE DUNE:2020jqi (right). We present the standard oscillation (black) and the probability obtained in our framework for or $\delta m = 10^{-4}~{\rm eV}$ (purple), $10^{-3}~{\rm eV}$ (dashed blue), $2\times 10^{-3}~{\rm eV}$ (dotted orange), and $5\times 10^{-3}~{\rm eV}$ (dot-dashed light blue). The inset indicate the biprobabilities $P(\bar{\nu}_\mu\to\bar{\nu}_e)$ vs $P(\nu_\mu\to\nu_e)$ for the same experiments together with current measurements in T2K and NO$\nu$A.
• Figure 5: Electron antineutrino survival probability $P(\bar{\nu}_e\to\bar{\nu}_e)$ as function of $L/E_{\nu}$ for standard oscillations (black) and our framework assuming $\delta m = 10^{-4}~{\rm eV}$ (purple), $10^{-3}~{\rm eV}$ (dashed blue), $2\times 10^{-3}~{\rm eV}$ (dotted orange), and $5\times 10^{-3}~{\rm eV}$ (dot-dashed light blue). The inset shows the Daya Bay region-of-interest together with the data points from Ref. DayaBay:2022orm. We also present the recent JUNO result JUNO:2025gmd as a green point.