Investigating DUNE oscillations sensitivity to sterile Pseudo-Dirac Neutrinos

TL;DR

The paper assesses Deep Underground Neutrino Experiment (DUNE) sensitivity to sterile neutrinos within a 3+(pseudo-Dirac pair) framework realized via the Linear-Inverse Seesaw (LISS). It develops a 5×5 mixing formalism with near- and far-detector analyses, identifying three Δm^2_{54} regimes (low, resonant, high) and showing that near-detector data can reveal persistent non-unitarity-like effects even in the low-mass limit due to keV-scale splittings. The study demonstrates that DUNE can significantly improve current constraints, particularly in appearance channels, and that new CP-violating phases in the sterile sector can either enhance or suppress signals by orders of magnitude. The results offer robust projections for probing LISS-like sterile neutrinos and guide experimental searches through ND–FD complementarity and phase-dependent effects.

Abstract

We explore the sensitivity of the Deep Underground Neutrino Experiment (DUNE) to sterile neutrino oscillations within a $3+$(pseudo-Dirac pair) framework. We first consider a pair of two sterile neutrinos forming a pseudo-Dirac pair, then we consider a low-scale seesaw realization, that we name ``Linear-Inverse Seesaw" model. This scenario features two nearly degenerate sterile neutrino states at the keV scale, characterized by a small mass splitting arising from a small amount of lepton number violation. In this scenario, the oscillation behavior can be described in three distinct regimes depending on the sterile-sterile mass-squared difference : low ($< 1\,\mathrm{eV}^2$), resonant ($1$--$100\,\mathrm{eV}^2$), and high ($> 100\,\mathrm{eV}^2$) regimes, recovering in both low- and high-mass regimes an effective non-unitarity of the leptonic mixing matrix. A distinctive feature of this framework is that observable effects persist even in the low-mass limit, unlike the case of standard $3+1$ scenarios, due to rapid oscillation averaging from larger keV-scale splittings. We leverage the complementarity of both near and far detectors to explore the sensitivity for $ν_e$ and $ν_μ$ disappearance and $ν_e$ and $ν_τ$ appearance oscillation probabilities. Our analysis reveals that DUNE can achieve significant improvements over current experimental constraints, especially in neutrino appearance modes. Additionally, we show that new CP-violating phases associated with the sterile sector can dramatically alter the sensitivity, with destructive interference potentially suppressing signals by orders of magnitude.

Figures (5)

• Figure 1: Muon neutrino disappearance (left) and electron neutrino appearance (right) probabilities as functions of energy for fixed active-sterile mixing parameters ($|U_{e4}|=|U_{e5}|=|U_{\mu 4}|=|U_{\mu 5}|=0.316$) and varying sterile neutrino mass splittings: $\Delta m_{54}^2 = 0.1$ eV$^2$ (green), $1$ eV$^2$ (red), $10$ eV$^2$ (blue), and $100$ eV$^2$ (orange). The energy range corresponds to more than 90$\%$ of the initial $\nu_\mu$ flux at the DUNE near detector.
• Figure 2: Allowed flavor structures in the LISS model consistent with neutrino oscillation data are shown for normal ordering (blue) and inverted ordering (red). Points with $U^4 > 1 \times 10^{-6}$ are highlighted in darker colors. Here, $U^2_{\alpha} \equiv U^2_{\alpha4} + U^2_{\alpha5}$ and $U^2 \equiv \sum_{\alpha} U^2_{\alpha}$.
• Figure 3: DUNE sensitivity to 3+pseudo-Dirac neutrino oscillations across all channels. (A): Electron neutrino disappearance sensitivity. (B): Muon neutrino disappearance sensitivity. (C): Electron neutrino appearance sensitivity probing $|U_{e4}U_{e5}U_{\mu4}U_{\mu5}|$. (D): and the analogous ones for tau neutrino appearance. Red contours show 90$\%$ C.L. exclusion regions for the Near Detector, while green contours correspond to the Far Detector. Blue dots represent allowed LISS model points for Inverted Ordering, and black triangles show Normal Ordering predictions. The points that will be probed by DUNE are shown in lighter colors. All results assume vanishing new CP phases ($\delta_{\text{CP}}^{ij} = 0$).
• Figure 4: DUNE sensitivity to new leptonic CP-violating phases in sterile neutrino oscillations. (A) and (B): Excluded regions in the $\Delta m_{54}^2$--$\delta_\text{CP}^{14}$ plane for electron neutrino appearance, with fixed values of $|U_{e4}U_{e5}U_{\mu4}U_{\mu5}|$ at $4.0\times10^{-6}$ (A) and $1.0\times10^{-5}$ (B). (C) and (D): Sensitivity reach for tau neutrino appearance in the $\Delta m_{54}^2$--$\delta_\text{CP}^{34}$ parameter space, with fixed $|U_{\tau4}U_{\tau5}U_{\mu4}U_{\mu5}|$ values of $2.0\times10^{-5}$ (C) and $5.0\times10^{-5}$ (D). The plots demonstrate how CP violating-phases can dramatically alter the experimental signature, with sensitivity varying by orders of magnitude across the phase space. All results correspond to 90$\%$ C.L. exclusion contours.
• Figure 5: Impact of new leptonic CP-violating phases on DUNE appearance channel sensitivity. (A): Near Detector constraints on electron neutrino appearance ($|U_{e4}U_{e5}U_{\mu4}U_{\mu5}|$ vs $\Delta m_{54}^2$) for three values of the CP-violating phase $\delta_\text{CP}^{14}$: $0$ (red), $\pi/2$ (green), and $\pi$ (blue). (B): Corresponding sensitivity for tau neutrino appearance ($|U_{\tau4}U_{\tau5}U_{\mu4}U_{\mu5}|$ vs $\Delta m_{54}^2$) with varying $\delta_\text{CP}^{34}$ phases. The dramatic variation between red and blue contours illustrates how CP-violating phases can either enhance or completely suppress the appearance signal, with constructive interference at $\delta_\text{CP} = 0$ yielding to the best sensitivity and destructive interference at $\delta_\text{CP} = \pi$ leading to strong suppression. All exclusion regions correspond to 90$\%$ C.L.