Measuring tau neutrino appearance probability via unitarity

We propose a unitarity method for determining tau neutrino appearance probability P(nu mu -> nu tau) in longbaseline (LBL) accelerator experiments and atmospheric neutrino observations. When simultaneous in situ measurements of P(nu mu -> nu mu) and P(nu mu -> nu e) proceed, as is typical in the LBL experiments, one can use unitarity to "measure" P(nu mu -> nu tau). A theorists' toy analysis for the model-independent determination of P(nu mu -> nu mu) and P(nu mu -> nu e) is presented by using the NOvA data. It is shown in our analysis that less than or similar to 5% (8%) measurement of tau neutrino appearance probability in neutrino (antineutrino) mode is possible in the peak region 1.5 less than or similar to E nu less than or similar to 2.5 GeV. The nu SM-independent nature of determination of the probabilities is emphasized.