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

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.

💡 Research Summary

The paper reports the first search for an eV‑scale sterile neutrino using data from the KM3NeT/ORCA detector, which at the time of the analysis was operating with only six detection units (ORCA6), corresponding to about 5 % of the final planned instrument. The data set comprises 5 828 up‑going atmospheric neutrino candidates collected between January 2020 and November 2021, amounting to an exposure of 433 kton·years. The analysis is performed within the 3 + 1 neutrino framework, where a fourth mass eigenstate (m₄) introduces an additional squared‑mass splitting Δm²₄₁ (assumed ≥ 1 eV²) and three new active‑sterile mixing angles. The authors focus on the mixing matrix elements U_{μ4} and U_{τ4}, which are directly related to the mixing angles θ₂₄ and θ₃₄.

Atmospheric neutrinos in the GeV energy range traverse the Earth’s interior before reaching the detector, experiencing sizable matter effects that modify the oscillation probabilities in the presence of a sterile state. The authors incorporate both charged‑current (V_CC) and neutral‑current (V_NC) potentials in a full four‑flavour Hamiltonian, and compute the ν_μ → ν_τ transition probability, which is most sensitive to the first oscillation maximum.

Event reconstruction exploits the Cherenkov light recorded by the digital optical modules (DOMs) to estimate neutrino energy and direction. A series of quality cuts, including topology‑based background rejection and up‑going selection, yields a clean sample with well‑understood detector response. Monte‑Carlo simulations based on GENIE for neutrino interactions and GEANT4 for photon propagation are used to model signal and background. Systematic uncertainties are introduced as nuisance parameters: atmospheric flux normalization (±10 %), energy scale (±5 %), angular resolution (±0.1 rad), Earth density profile variations, DOM efficiency, and optical background noise. Gaussian priors are assigned to each nuisance parameter.

The statistical analysis employs a binned Poisson likelihood across energy–zenith bins, profiled over the nuisance parameters, to construct a χ² function. A two‑dimensional scan in the (|U_{μ4}|², |U_{τ4}|²) plane yields the best‑fit point (|U_{μ4}|² ≈ 0.03, |U_{τ4}|² ≈ 0.02) and the corresponding confidence intervals. No significant deviation from the standard three‑flavour hypothesis is observed. At the 90 % confidence level the limits are |U_{μ4}|² < 0.138 and |U_{τ4}|² < 0.076 for Δm²₄₁ ≥ 1 eV². These constraints are compatible with, and slightly less stringent than, those obtained by IceCube‑DeepCore, MINOS+, NOvA, and other long‑baseline experiments, which is expected given the modest size of the data set.

The authors emphasize that the current limits are derived from only 5 % of the final detector. Simulations indicate that the full ORCA configuration (∼100 % of the planned 115 detection units) would increase the event statistics by roughly a factor of twenty and reduce systematic uncertainties through improved calibration. Consequently, future ORCA data could push the limits on |U_{μ4}|² and |U_{τ4}|² down to the O(10⁻²) level or better, providing a powerful complement to short‑baseline reactor and accelerator searches, especially for the τ‑flavour mixing which is poorly constrained by other experiments.

In conclusion, this work demonstrates that even a partially‑built KM3NeT/ORCA can deliver competitive sterile‑neutrino searches. The analysis validates the detector’s capability to reconstruct atmospheric neutrino oscillations with sufficient precision to probe new physics, and it sets the stage for ORCA to become a key player in the global effort to resolve the eV‑scale sterile neutrino hypothesis.