Final NOMAD results on nu_mu->nu_tau and nu_e->nu_tau oscillations including a new search for nu_tau appearance using hadronic tau decays
NOMAD Collaboration, P. Astier
TL;DR
The paper addresses the search for $\nu_\mu \rightarrow \nu_\tau$ and $\nu_e \rightarrow \nu_\tau$ oscillations in the NOMAD data, employing a unified hadronic $\tau$ decay analysis to boost sensitivity. It combines this improved hadronic-$\tau$ search with the $\nu_\tau e\bar{\nu}_e$ DIS channel and LM topologies to present the overall NOMAD limits, achieving roughly a factor of two improvement in sensitivity with reduced systematic uncertainties. No evidence for oscillations is observed, leading to 90% confidence level regions in the $(\sin^2(2\theta),\Delta m^2)$ plane: for $\nu_\mu \rightarrow \nu_\tau$, $\sin^2(2\theta) < 3.3 \times 10^{-4}$ at large $\Delta m^2$ and $\Delta m^2 < 0.7$ eV$^2/c^4$ at $\sin^2(2\theta)=1$; for $\nu_e \rightarrow \nu_\tau$, $\sin^2(2\theta) < 1.5 \times 10^{-2}$ at large $\Delta m^2$ and $\Delta m^2 < 5.9$ eV$^2/c^4$ at $\sin^2(2\theta)=1$. The analysis also sets limits on effective couplings of the tau lepton to $\nu_\mu$ or $\nu_e$ and demonstrates the impact of the hadronic-$\tau$ unification on the NOMAD sensitivity.
Abstract
Results from the nu_tau appearance search in a neutrino beam using the full NOMAD data sample are reported. A new analysis unifies all the hadronic tau decays, significantly improving the overall sensitivity of the experiment to oscillations. The "blind analysis" of all topologies yields no evidence for an oscillation signal. In the two-family oscillation scenario, this sets a 90% C.L. allowed region in the sin^2(2theta)-Delta m^2 plane which includes sin^2(2theta)<3.3 x 10^{-4} at large Delta m^2 and Delta m^2 < 0.7 eV^2/c^4 at sin^2(2theta)=1. The corresponding contour in the nu_e->nu_tau oscillation hypothesis results in sin^2(2theta)<1.5 x 10^{-2} at large Delta m^2 and Delta m^2 < 5.9 eV^2/c^4 at sin^2(2theta)=1. We also derive limits on effective couplings of the tau lepton to nu_mu or nu_e.