A bstract 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 → ν τ standard oscillation maximum, simultaneous constraints are put on the magnitude of the U μ 4 and U τ 4 mixing elements assuming m₄₁² 1 Δ m 41 2 ≥ 1 eV 2. The results are compatible with the absence of mixing between active neutrinos and a sterile state, with | U μ 4 | 2 < 0. 138 and | U τ 4 | 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.
Adriani et al. (2026) studied this question.