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

TL;DR

This paper reports the first measurements of tau neutrino interactions with the KM3NeT/ORCA detector, including a cross section measurement and improved limits on non-unitary neutrino mixing, using early detector data.

Contribution

It presents the first analysis of tau neutrino detection and non-unitarity constraints with the KM3NeT/ORCA detector using initial data from six detection units.

Findings

Measured tau neutrino normalization as 0.48 with uncertainties.

Estimated tau neutrino charged-current cross section at 20.3 GeV.

Set a new limit on the non-unitarity parameter, α₃₃ > 0.95 at 95% CL.

Abstract

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 $\nu_\tau$ normalisation, defined as the ratio between the number of observed and expected tau neutrino events, is measured to be $S_\tau = 0.48^{+0.5}_{-0.33}$. This translates into a $\nu_\tau$ charged-current cross section measurement of $\sigma_{\tau}^{\text{meas}} = (2.5 ^{+2.6}_{-1.8}) \times 10^{-38}$ cm$^{2}$ nucleon$^{-1}$ at the median $\nu_\tau$ 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 $\tau$-row of the neutrino mixing matrix was improved, with $\alpha_{33}>$ 0.95 at the 95$\%$ confidence level.