Measuring tau neutrino appearance probability via unitarity

Abstract

We propose a {\em unitarity method} for determining $\tau$ neutrino appearance probability $P({\nu }_{\mu }\to {\nu }_{\tau })$ in long-baseline (LBL) accelerator experiments and atmospheric neutrino observations. When simultaneous in situ measurements of $P({\nu }_{\mu }\to {\nu }_{\mu })$ and $P({\nu }_{\mu }\to {\nu }_e)$ proceed, as is typical in the LBL experiments, one can use unitarity to "measure" $P({\nu }_{\mu }\to {\nu }_{\tau })$. A theorists' toy analysis for the model-independent determination of $P({\nu }_{\mu }\to {\nu }_{\mu })$ and $P({\nu }_{\mu }\to {\nu }_e)$ is presented by using the NOvA data. It is shown in our analysis that $\text{lsim}$5\% (8\%) measurement of $\tau$ neutrino appearance probability in neutrino (antineutrino) mode is possible in the peak region $1.5\lesssim E_{\nu }\lesssim 2.5$ GeV. The $\nu$SM-independent nature of determination of the probabilities is emphasized.

Comments: Published version. New references added. 22 pages, 5 figures