First muon-neutrino disappearance study with an off-axis beam

We report a measurement of muon-neutrino disappearance in the T2K experiment. The 295-km muon-neutrino beam from Tokai to Kamioka is the first implementation of the off-axis technique in a long-baseline neutrino oscillation experiment. With data corresponding to $1.43\times {10}^{20}$ protons on target, we observe 31 fully-contained single $\mu$-like ring events in Super-Kamiokande, compared with an expectation of $104\pm 14(\mathrm{syst})$ events without neutrino oscillations. The best-fit point for two-flavor ${\nu }_{\mu }\to {\nu }_{\tau }$ oscillations is ${sin}^2(2{\theta }_{23})=0.98$ and $|\Delta m_{32}^2|=2.65\times {10}^{-3}{\mathrm{eV}}^2$. The boundary of the 90% confidence region includes the points $({sin}^2(2{\theta }_{23}),|\Delta m_{32}^2|)=(1.0,3.1\times {10}^{-3}{\mathrm{eV}}^2)$, (0.84, $2.65\times {10}^{-3}{\mathrm{eV}}^2$) and (1.0, $2.2\times {10}^{-3}{\mathrm{eV}}^2$).

Figure 1

(Top) the predicted flux of ${\nu }_{\mu }$ as a function of neutrino energy without oscillations at Super-Kamiokande and at the off-axis near detector; (bottom) the flux of ${\nu }_{\mu }$ and ${\overline{\nu }}_{\mu }$ at Super-Kamiokande. The shaded boxes indicate the total systematic uncertainty for each energy bin.

Figure 2

Neutrino energy reconstructed for the CCQE hypothesis for ${\nu }_{\mu }$ CC candidates interacting in the FGD target. The data are shown using points with error bars (statistical only), and the MC predictions are in shaded histograms.

Figure 3

Reconstructed energy spectrum of the 31 data events compared with the expected spectra in the far detector without disappearance and with best-fit ${\nu }_{\mu }\to {\nu }_{\tau }$ oscillations. A variable binning scheme is used here for the purpose of illustration only; the actual analysis used equal-sized 50 MeV bins.

Figure 4

The 90% confidence regions for ${sin}^2(2{\theta }_{23})$ and $|\Delta m_{32}^2|$; results from the two analyses reported here are compared with those from MINOS [5] and Super-Kamiokande [6, 31].