New oscillation and scattering constraints on the tau row matrix elements without assuming unitarity

TL;DR

This paper improves constraints on the tau neutrino row of the lepton mixing matrix by analyzing data involving tau neutrinos, especially atmospheric neutrinos, without assuming unitarity, and discusses future experimental sensitivities.

Contribution

It provides new constraints on the tau neutrino row elements of the mixing matrix without assuming unitarity, using existing data and projections for upcoming experiments.

Findings

Tau neutrino row constraints are improved without assuming unitarity.

Existing data involving tau neutrinos can inform the mixing matrix.

Future experiments could achieve tau row precision comparable to muon row.

Abstract

The tau neutrino is the least well measured particle in the Standard Model. Most notably, the tau neutrino row of the lepton mixing matrix is quite poorly constrained when unitarity is not assumed. In this paper, we identify data sets involving tau neutrinos that improve our understanding of the tau neutrino part of the mixing matrix, in particular $\nu_\tau$ appearance in atmospheric neutrinos. We present new results on the elements of the tau row leveraging existing constraints on the electron and muon rows for the cases of unitarity violation, with and without kinematically accessible steriles. We also show the expected sensitivity due to upcoming experiments and demonstrate that the tau neutrino row precision may be comparable to the muon neutrino row in a careful combined fit.