Study of tau neutrinos and non-unitary neutrino mixing with the first six detection units of KM3NeT/ORCA

Oscillations of atmospheric muon and electron neutrinos produce tau neutrinos with energies in the GeV range, which can be observed by the ORCA detector of the KM3NeT neutrino telescope in the Mediterranean Sea. First measurements with ORCA6, an early subarray corresponding to about 5% of the final detector, are presented. A sample of 5828 neutrino candidates has been selected from the analysed exposure of 433 kton-years. The ντ normalisation, defined as the ratio between the number of observed and expected tau neutrino events, is measured to be S_τ = 0.48^+0.5_-0.33. This translates into a ντ charged-current cross section measurement of σ^meas_τ = (.2.5^+2.6_-1.8) x 10^-38 cm² nucleon⁻¹ at the median ντ energy of 20.3 GeV. The result is consistent with the measurements of other experiments. In addition, the current limit on the non-unitarity parameter affecting the τ-row of the neutrino mixing matrix was improved, with α33 > 0.95 at the 95% confidence level.