Measuring tau neutrino appearance probability via unitarity

TL;DR

This paper introduces a unitarity-based method to measure tau neutrino appearance probability in long-baseline and atmospheric experiments, enabling model-independent and precise determination using existing data.

Contribution

The paper proposes a novel unitarity method for model-independent measurement of tau neutrino appearance probability in neutrino experiments.

Findings

Potential to measure tau neutrino appearance probability with less than 5% uncertainty in neutrino mode.

Achieves approximately 8% measurement accuracy in antineutrino mode.

Applicable to current NOvA data and relevant for future neutrino oscillation studies.

Abstract

We propose a {\em unitarity method} for determining $\tau$ neutrino appearance probability $P(\nu_{\mu} \rightarrow \nu_{\tau})$ in long-baseline (LBL) accelerator experiments and atmospheric neutrino observations. When simultaneous in situ measurements of $P(\nu_{\mu} \rightarrow \nu_{\mu})$ and $P(\nu_{\mu} \rightarrow \nu_{e})$ proceed, as is typical in the LBL experiments, one can use unitarity to "measure" $P(\nu_{\mu} \rightarrow \nu_{\tau})$. A theorists' toy analysis for the model-independent determination of $P(\nu_{\mu} \rightarrow \nu_{\mu})$ and $P(\nu_{\mu} \rightarrow \nu_{e})$ is presented by using the NOvA data. It is shown in our analysis that $\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.