Search for an eV-scale sterile neutrino with the first six detection units of KM3NeT/ORCA

TL;DR

This paper reports the first search for an eV-scale sterile neutrino using KM3NeT/ORCA data from the ORCA6 configuration (6 detection units, 433 kton-years). By analyzing νμ→ντ oscillations in atmospheric neutrinos and employing a 3+1 framework with Δm^2_{41} = 1 eV^2, the authors constrain the active–sterile mixing elements to |U_{μ4}|^2 < 0.138 and |U_{τ4}|^2 < 0.076 at 90% CL, finding results compatible with no sterile mixing. A detailed likelihood-based analysis incorporates extensive nuisance parameters and uses reconstructed energy and zenith-angle distributions across track and shower event classes. The findings are competitive with existing measurements and demonstrate KM3NeT/ORCA’s capability to contribute to sterile-neutrino searches, with future prospects including sensitivity to TeV-scale matter-induced resonances in the final detector configuration. The study establishes a robust framework for combining detailed detector response and oscillation modeling to probeBeyond-Standard-Model neutrino scenarios in next-generation atmospheric neutrino experiments.

Abstract

The existence of an eV-scale sterile neutrino has been proposed to explain several anomalous experimental results obtained over the course of the past 25 years. The first search for such a sterile neutrino conducted with data from KM3NeT/ORCA -- a water Cherenkov neutrino telescope under construction at the bottom of the Mediterranean Sea -- is reported in this paper. GeV-scale atmospheric neutrino oscillations are measured by reconstructing the energy and arrival direction of up-going neutrinos that have traversed the Earth. This study is based on a data sample containing 5828 neutrino candidates collected with 6 detection units ($5\%$ of the complete detector), corresponding to an exposure of 433 kton-years. From the expected effect of an eV-scale sterile neutrino on the first $ν_μ\rightarrow ν_τ$ standard oscillation maximum, simultaneous constraints are put on the magnitude of the $U_{μ4}$ and $U_{τ4}$ mixing elements assuming $Δm^2_{41} \geq 1$ eV$^2$. The results are compatible with the absence of mixing between active neutrinos and a sterile state, with $|U_{μ4}|^2 < 0.138$ and $|U_{τ4}|^2 < 0.076$ at a $90\%$ confidence level. Such constraints are compatible with the results reported by other long-baseline experiments, and indicate that with KM3NeT/ORCA it is possible to bring crucial contributions to sterile neutrino searches in the coming years.

Figures (6)

• Figure 1: Difference in muon neutrino (left) and antineutrino (right) survival probability between the standard three-flavour model and a 3$+$1 model with $\Delta m^2_{41} = 1$ eV2 and $\sin^2 \theta_{24} = 0.1$ for up-going neutrinos traveling through the Earth. Normal ordering is assumed and the standard oscillation parameter values are fixed to the NuFit 5.2 ones estebanFateHintsUpdated2020. The other sterile parameters are set to $0$. The L/E lines are shown in units of km/GeV. The oscillograms presented in this article are all computed with OscProb joao_coelho_2023_10104847 using the Preliminary Reference Earth Model dziewonskiPreliminaryReferenceEarth1981.
• Figure 2: Muon neutrino survival probability as a function of $E_\nu$, for vertically up-going neutrinos ($\cos \theta = -1.0$). Normal ordering is assumed except when mentioned otherwise, with NuFit 5.2 values estebanFateHintsUpdated2020 for the standard oscillation parameters. The three-flavour model (dashed black curve) is compared with various 3$+$1 models all assuming $\Delta m^2_{41} = 1$ eV2. Left: different values of $\theta_{24}$ and $\theta_{34}$ ($s^2_{ij} = \sin^2\theta_{ij}$), with $\delta_{24}$ and other active-sterile mixing parameters fixed to 0. Right: different $\delta_{24}$ values, with $\sin^2 \theta_{24} = 0.07$, $\sin^2 \theta_{34} = 0.1$ and other active-sterile mixing parameters fixed to 0.
• Figure 3: Left: Log-likelihood ratio map obtained from the ORCA6 data sample over the $|U_{\mu 4}|^2$ and $|U_{\tau 4}|^2$ phase space. $-2\Delta \log \mathcal{L}$ is not displayed when higher than $10$ units. The black lines show upper limits of the allowed parameter space at various confidence levels. Right: Comparison of the observed and expected upper limits at $90\%$ CL on the $U_{\mu 4}$ and $U_{\tau 4}$ mixing elements for ORCA6. The excluded regions are on the top right side of the contours.
• Figure 4: Event distributions in $L/E$ for each class of the ORCA6 data (black), compared with the model prediction at the best-fit point in the eV sterile neutrino case (red) and in the standard oscillation case (dashed blue). The dashed orange curve shows the model prediction for an arbitrary point in the $(|U_{\mu 4}|^2, |U_{\tau 4}|^2)$ phase space.
• Figure 5: Muon neutrino survival probability as a function of the neutrino energy under various scenarios for $\cos \theta = -1.0$. The three-flavour standard oscillation model assuming the nominal value for $\theta_{23}=49.1\degree$ (dashed black) is compared with the three-flavour model with $\theta_{23}=59\degree$ (dotted black) and with the 3$+$1 models with $\sin^2 \theta_{24} = 0.1$ (blue) and $\sin^2 \theta_{34} = 0.15$ (orange), for $\Delta m_{41}^2=1$ eV2. All other sterile parameters are set to 0.