Evidence for Muon Neutrino Oscillation in an Accelerator-Based Experiment

We present results for ${\nu }_{\mu }$ oscillation in the KEK to Kamioka (K2K) long-baseline neutrino oscillation experiment. K2K uses an accelerator-produced ${\nu }_{\mu }$ beam with a mean energy of 1.3 GeV directed at the Super-Kamiokande detector. We observed the energy-dependent disappearance of ${\nu }_{\mu }$, which we presume have oscillated to ${\nu }_{\tau }$. The probability that we would observe these results if there is no neutrino oscillation is 0.0050% ($4.0\sigma$).

Figure 1

The $q_{\mathrm{r}\mathrm{e}\mathrm{c}}^2$ distributions for 2-track non-QE samples of SciFi (left) and SciBar (right). Open circles, solid lines, and hatched areas show data, MC predictions, and the CC-QE component estimated from MC simulation, respectively.

Figure 2

A selection of muon momentum ($p_{\mu }$) and direction (${\theta }_{\mu }$) distributions: (a) the $p_{\mu }$ distribution of the 1KT fully contained 1-ring $\mu$-like sample, (b) 1KT ${\theta }_{\mu }$ for the same sample, (c) SciFi $p_{\mu }$ for the 2-track QE sample, and (d) SciBar ${\theta }_{\mu }$ for the 2-track non-QE sample. Open circles represent data, while histograms are MC predictions using the best fit $E_{\nu }$ spectrum and suppression of the resonant pion production.

Figure 3

The reconstructed $E_{\nu }$ distribution for the SK 1-ring $\mu$-like sample. Points with error bars are data. The solid line is the best fit spectrum. The dashed line is the expected spectrum without oscillation. These histograms are normalized by the number of events observed (57).

Figure 4

Allowed regions of oscillation parameters. Dashed, solid, and dot-dashed lines are 68.4%, 90% and 99% C.L. contours, respectively.