Revealing Neutrino Mass Ordering at CEPC and FCC-ee

Abstract

The neutrino masses ordering remains one of the most important open questions in neutrino physics. While upcoming oscillation experiments aim to resolve this problem at low energies, complementary approaches are highly desirable. In this Letter, we show that the neutrino mass ordering can be probed at high-energy colliders through the lepton-flavor structure of heavy neutral lepton (HNL) interactions. In the minimal Type-I seesaw scenario with two nearly degenerate HNLs, the heavy--light neutrino mixings are strongly correlated with the light-neutrino mass spectrum, leading to distinct flavor patterns for the normal and inverted hierarchies. We demonstrate that future $Z$ factories, such as CEPC and FCC-ee, can probe the neutrino mass ordering for total HNL mixings as small as $U_{\rm tot}^2 \gtrsim 4 \times 10^{-9}$, and discriminate between the two hierarchies for $U_{\rm tot}^2 \gtrsim 10^{-6}$. Our results establish collider searches for HNLs as a powerful and complementary probe of the neutrino mass ordering.

Figures (3)

• Figure 1: Ternary plot showing the distribution of flavor-mixing ratios $U^2_\alpha/U^2_{tot}$, $\alpha=e,\mu,\tau$ for NH (blue) and IH (green). They are obtained by NuFIT 6.0 Esteban:2024eliNufit and JUNO's new result JUNO:2025gmd. The dashed red (brown) curve shows the region under the best-fit CP phase from T2K (NO$\nu$A), assuming a future uncertainty of $8^\circ$ ($7^\circ$) from DUNE+$\mu$THEIA Ge:2022iac.
• Figure 2: Sensitivity of the neutrino mass orderings in the plane of $(U_{e N}^2/U_{\rm tot}^2, U_{\mu N}^2/U_{\rm tot}^2)$, at $\sqrt{s}=$ 91.2 GeV CEPC/FCC-ee with $\mathcal{L} \approx 200~ab^{-1}$, for $U^2_{\rm tot} \approx 10^{-5}$ (a), $10^{-7}$ (b), $10^{-8}$ (c), and $4 \times 10^{-9}$ (d), respectively, when $m_N \sim 50$ GeV.
• Figure 3: Fraction of the parameter space (in $\log_{10}$ scale) that allows unambiguous identification of the neutrino mass ordering (NMO) via $e^+e^- \to N \nu\,(\bar{\nu})$, followed by $N \to \ell_\alpha j j$, at $\sqrt{s}=91.2$ GeV for CEPC/FCC-ee with $\mathcal{L}\simeq 200~\mathrm{ab}^{-1}$. Regions with $K>1\%~(30\%,\,60\%,\,90\%)$ and $K<30\%~(60\%,\,90\%)$ are shaded in red (green, orange, and cyan), respectively. The red area labeled "Current" is excluded by existing constraints Bolton:2019pcu, while dashed contours labeled "SHiP" and "CMS" indicate future sensitivities Alekhin:2015byhDrewes:2019fou. The black line denotes the bound from $\ell_\alpha\to \ell_\beta\gamma$, for NH (dashed) and IH (solid). The "Seesaw" region reflects the cosmological bound $\sum_i m_{\nu_i} \simeq U_{\rm tot}^2\, m_N \lesssim 0.12~\mathrm{eV}$ with $i=1,2,3$ from Planck data Planck:2018vyg.