Evidence for oscillation of atmospheric neutrinos

Abstract

We present an analysis of atmospheric neutrino data from a 33.0 kiloton-year (535-day) exposure of the Super-Kamiokande detector. The data exhibit a zenith angle dependent deficit of muon neutrinos which is inconsistent with expectations based on calculations of the atmospheric neutrino flux. Experimental biases and uncertainties in the prediction of neutrino fluxes and cross sections are unable to explain our observation. The data are consistent, however, with two-flavor nu_mu <-> nu_tau oscillations with sin^2(2theta)>0.82 and 5x10^-4 < delta m^2 < 6x10^-3 eV^2 at 90% confidence level.

Figures (4)

• Figure 1: The $(U-D)/(U+D)$ asymmetry as a function of momentum for FC $e$-like and $\mu$-like events and PC events. While it is not possible to assign a momentum to a PC event, the PC sample is estimated to have a mean neutrino energy of 15 GeV. The Monte Carlo expectation without neutrino oscillations is shown in the hatched region with statistical and systematic errors added in quadrature. The dashed line for $\mu$-like is the expectation for $\nu_{\rm \mu} \leftrightarrow {\nu}_{\rm \tau}$ oscillations with $(\sin^2 2\theta=1.0, \Delta m^2 = 2.2 \times 10^{-3}$ eV$^2$).
• Figure 2: The 68%, 90% and 99% confidence intervals are shown for $\sin^2 2\theta$ and $\Delta m^2$ for $\nu_{\rm \mu} \leftrightarrow {\nu}_{\rm \tau}$ two-neutrino oscillations based on 33.0 kiloton-years of Super--Kamiokande data. The 90% confidence interval obtained by the Kamiokande experiment is also shown.
• Figure 3: Zenith angle distributions of $\mu$-like and $e$-like events for sub-GeV and multi-GeV data sets. Upward-going particles have $\cos \Theta < 0$ and downward-going particles have $\cos \Theta > 0$. Sub-GeV data are shown separately for $p < 400$ MeV$/c$ and $p > 400$ MeV$/c$. Multi-GeV $e$-like distributions are shown for $p < 2.5$ GeV$/c$ and $p > 2.5$ GeV$/c$ and the multi-GeV $\mu$-like are shown separately for FC and PC events. The hatched region shows the Monte Carlo expectation for no oscillations normalized to the data live-time with statistical errors. The bold line is the best-fit expectation for $\nu_{\rm \mu} \leftrightarrow {\nu}_{\rm \tau}$ oscillations with the overall flux normalization fitted as a free parameter.
• Figure 4: The ratio of the number of FC data events to FC Monte Carlo events versus reconstructed $L/E_\nu$. The points show the ratio of observed data to MC expectation in the absence of oscillations. The dashed lines show the expected shape for $\nu_{\rm \mu} \leftrightarrow {\nu}_{\rm \tau}$ at $\Delta m^2=2.2\times10^{-3}$eV$^2$ and $\sin^2 2\theta=1$. The slight $L/E_\nu$ dependence for $e$-like events is due to contamination (2-7%) of $\nu_\mu$ CC interactions.