Measurement of Atmospheric Neutrino Flux Consistent with Tau Neutrino Appearance

A search for the appearance of tau neutrinos from ${\nu }_{\mu }\leftrightarrow {\nu }_{\tau }$ oscillations in the atmospheric neutrinos has been performed using 1489.2 days of atmospheric neutrino data from the Super-Kamiokande-I experiment. A best fit tau neutrino appearance signal of $138\pm 48(\mathrm{stat}{)}_{-32}^{+15}(\mathrm{syst})$ events is obtained with an expectation of $78\pm 26(\mathrm{syst})$. The hypothesis of no tau neutrino appearance is disfavored by 2.4 sigma.

Figure 1

The distributions of variables: (a) Visible energy, (b) maximum distance between the primary interaction and electron vertices from pions and then $\mu$ decays, (c) number of ring candidates, (d) sphericity in the laboratory frame, and (e) clustered sphericity in the center of mass, after applying the ${\nu }_{\tau }$ event selection criteria for downward-going data (points), ${\nu }_{\tau }$ MC events (shaded histogram), and atmospheric ${\nu }_e{,}_{\mu }$ background MC events (solid histogram). (The histograms of ${\nu }_{\tau }$ MC events are normalized arbitrarily.) In the likelihood analysis, the data sample is divided into 5 energy bins.

Figure 2

The likelihood (top) and NN output (bottom) distributions of downward-going (left) and upward-going (right) events for data (points), atmospheric neutrino background (BKG) MC events (dashed histogram), and the best fit including tau neutrino and backgrounds (solid histogram). The shaded area shows a fitted excess of tau neutrino events in the upward-going direction. The events for likelihood $\mathcal{L}>0$ or NN output $>0.5$ are defined to be taulike.

Figure 3

The zenith angle distributions for the likelihood (top) and neural network (bottom) analyses. Zenith angle $\cos \theta =-1$ ($\cos \theta =+1$) indicates upward-going (downward-going) direction. The data sample is fitted after ${\nu }_{\tau }$ event selection criteria are applied. The solid histogram shows the best fit including ${\nu }_{\tau }$, and the dashed histogram shows the backgrounds from atmospheric neutrinos (${\nu }_e$ and ${\nu }_{\mu }$). A fitted excess of taulike events in the upward-going direction is shown in the shaded area.