Learning from τ appearance

The study of νμ→ντ oscillations and the explicit observation of ντ through the identification of the final-state τ lepton ('direct appearance search') represent the most straightforward test of the oscillation phenomenon. It is, nonetheless, the most challenging from the experimental point of view. In this paper, we discuss current empirical evidence for the direct appearance of tau neutrinos at the atmospheric scale and the perspectives for the next few years, up to the completion of the CNGS physics programme. We investigate the relevance of this specific oscillation channel for gaining insights into neutrino physics within the standard three-family framework. Finally, we discuss the opportunities offered by precision studies of νμ→ντ transitions for the occurrence of more exotic scenarios, emerging from additional sterile neutrinos or non-standard interactions.