Evidence for an Oscillatory Signature in Atmospheric Neutrino Oscillations

Muon neutrino disappearance probability as a function of neutrino flight length $L$ over neutrino energy $E$ was studied. A dip in the $L/E$ distribution was observed in the data, as predicted from the sinusoidal flavor transition probability of neutrino oscillation. The observed $L/E$ distribution constrained ${\nu }_{\mu }\leftrightarrow {\nu }_{\tau }$ neutrino oscillation parameters; $1.9\times {10}^{-3}<\Delta m^2<3.0\times {10}^{-3}{\mathrm{e}\mathrm{V}}^2$ and ${sin}^{2}2\theta >0.90$ at 90% confidence level.

Figure 1

Observed number of photoelectrons in the OD divided by the expectation from the projected track length of the muon for the data (points), the OD through-going MC events (white region in histogram) and the OD stopping MC events (hatched region). The MC does not include oscillations and is normalized by the live time. If oscillation is included in the MC, the normalization of the data and MC agrees within 10%.

Figure 2

Contour plots of 70% $L/E$ resolution in the ($\cos \Theta$, $E_{\nu }$) plane for (a) FC single-ring, (b) FC multiring, (c) PC OD stopping, and (d) PC OD through-going samples. Three additional lines in (a) show the survival probabilities of muon neutrinos predicted from neutrino oscillation with $({sin}^{2}2\theta ,\Delta m^2)=(1.00,2.4\times {10}^{-3}{\mathrm{e}\mathrm{V}}^2)$. Full and half oscillations occur on the solid and dashed lines, respectively.

Figure 3

Number of events as a function of $L/E$ for the data (points) and the atmospheric neutrino MC events without oscillations (histogram). The MC is normalized by the detector live time.

Figure 4

Ratio of the data to the MC events without neutrino oscillation (points) as a function of the reconstructed $L/E$ together with the best-fit expectation for 2-flavor ${\nu }_{\mu }\leftrightarrow {\nu }_{\tau }$ oscillations (solid line). The error bars are statistical only. Also shown are the best-fit expectation for neutrino decay (dashed line) and neutrino decoherence (dotted line).

Figure 5

68%, 90%, and 99% C.L. allowed oscillation parameter regions for 2-flavor ${\nu }_{\mu }\leftrightarrow {\nu }_{\tau }$ oscillations obtained by the present analysis.