Sensitivity to atmospheric tau-neutrino appearance and all-flavour search for neutrinos from the Fermi Bubbles with the deep-sea telescopes KM3NeT/ORCA and ANTARES

Neutrino telescopes are used to search for the sources of astrophysical neutrinos and to study the fundamental properties of neutrinos. They come in three flavours - electron, muon and tau neutrinos. While muon neutrinos produce distinct elongated 'track-like' light signatures in the telescopes, electron and tau neutrinos appear predominantly 'shower-like'. At TeV to PeV energies relevant for astrophysical source searches, neutrinos of both signatures can be selected out of the dominant backgrounds based on a few discriminating variables. Such a so-called 'cut and count' analysis for track- and shower-like signatures has been performed using eight years of data recorded with the ANTARES telescope and an upper limit on the neutrino flux from the 'Fermi Bubbles', an extended region of gamma-ray emission in the central part of our Galaxy, has been set. The KM3NeT/ORCA detector is under construction and optimised to study neutrino properties in the few-GeV energy range. Since differences between the event signatures at these energies are more subtle, classification models using 'Random Decision Forests' have been employed to identify neutrino candidates and to separate tracks from showers. With the resulting models, the oscillation from (mainly) atmospheric muon neutrinos into the tau flavour can be measured on a statistical basis. The sensitivity of the detector to measure this tau-neutrino appearance has been studied. It is found that after one year of operation, KM3NeT/ORCA will determine the strength of the tau- neutrino flux to within ±30% at 3σ CL. This measurement precision allows to constrain the currently large theoretical uncertainties on the tau-neutrino cross sections and to test whether the generally accepted picture of flavour oscillation amongst the three known active neutrino flavours is complete.