Search for short baseline ${\nu }_e$ disappearance with the T2K near detector

The T2K experiment has performed a search for ${\nu }_e$ disappearance due to sterile neutrinos using $5.9\times 10^{20}$ protons on target for a baseline of 280 m in a neutrino beam peaked at about 500 MeV. A sample of ${\nu }_e$ CC interactions in the off-axis near detector has been selected with a purity of 63% and an efficiency of 26%. The p-value for the null hypothesis is 0.085 and the excluded region at 95% C.L. is approximately ${\sin }^{2}2{\theta }_{\text{ee}}>0.3$ for $\mathrm{\Delta }{m}_{\text{eff}}^2>7{\mathrm{eV}}^2/{\mathrm{c}}^4$.

Figure 1

Expected ${\nu }_e$ flux at ND280 and CC ${\nu }_e$ selection efficiency as a function of the true neutrino energy are shown.

Figure 2

Expected neutrino flight path for ${\nu }_e$ interacting in the ND280 FV, broken down by the neutrino parent meson.

Figure 3

Reconstructed energy distributions of the ${\nu }_e$ (top) and control (bottom) samples. The distributions are broken down by ${\nu }_e$ interactions (signal), background inside the fiducial volume due to ${\nu }_{\mu }N\to {\pi }^{0}X$ (In-FV ${\nu }_{\mu }N\to {\pi }^{0}X$), background outside the fiducial volume due to ${\nu }_{\mu }N\to {\pi }^{0}X$ (OOFV ${\nu }_{\mu }N\to {\pi }^{0}X$) and all other sources of background (${\nu }_{\mu }$ other). Both $\nu$ and $\overline{\nu }$ are included in the samples. The ratio of the data to the MC expectation in the null oscillation hypothesis is shown for both samples. The red error band corresponds to the fractional systematic uncertainty. Black dots represent the data with the statistical uncertainty.

Figure 4

The ratio of the best-fit spectrum to the expected MC distribution, where the fit includes nuisance and oscillation parameters (blue) and nuisance parameters only (red dashed), is shown. The plots show the ${\nu }_e$ sample (top) and the control sample (bottom). The black line corresponds to the expected nonoscillated MC before the fit. The black dots show the data. Statistical uncertainties are shown.

Figure 5

68% and 90% C.L. allowed regions and 95% C.L. exclusion region for the ${\sin }^{2}2{\theta }_{\text{ee}}\text{−}\mathrm{\Delta }{m}_{\text{eff}}^2$ parameters measured with the T2K near detector.

Figure 6

The T2K confidence interval in the ${\sin }^{2}2{\theta }_{\text{ee}}\text{−}\mathrm{\Delta }{m}_{\text{eff}}^2$ parameter space at 90% C.L. (top) and 95% C.L. (bottom) is compared with the other experimental results available in literature: allowed regions of gallium and reactor anomalies and excluded regions by ${\nu }_e$-carbon interaction data and solar neutrino data [14]. The T2K best fit is marked by a green star; the best fit of other experimental results corresponds to circles of the same coloring as the limits. In the region $\mathrm{\Delta }{m}_{\text{eff}}^2<0.2{\mathrm{eV}}^2/{\mathrm{c}}^4$ limits on the mixing angle ${\sin }^{2}2{\theta }_{\text{ee}}$ have also been set by the Daya Bay experiment [48].