Tau appearance in atmospheric neutrino interactions

If the correct interpretation of the Super-Kamiokande atmospheric neutrino data is νμ→ντ oscillation, the contained data sample should already have more than 10 τ appearance events. We study the challenging task of detecting the τ, focussing on the decay chain τ±→ρ±→π±π0 in events with quasi-elastic τ production. The background level, which is currently quite uncertain because of a lack of relevant neutral current data, can be measured by the near detector in the K2K experiment. Our estimates of the background suggest that it may be possible to detect τ appearance in Super-Kamiokande with 5–10 years of running.