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

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 $$ \Delta {m}_{41}^2\ge 1 $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>Δ</mml:mi> <mml:msubsup> <mml:mi>m</mml:mi> <mml:mn>41</mml:mn> <mml:mn>2</mml:mn> </mml:msubsup> <mml:mo>≥</mml:mo> <mml:mn>1</mml:mn> </mml:math> eV 2 . The results are compatible with the absence of mixing between active neutrinos and a sterile state, with | U μ 4 | 2 &lt; 0.138 and | U τ 4 | 2 &lt; 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.