Search for Muon-Neutrino to Electron-Neutrino Transitions in MINOS

This Letter reports on a search for ${\nu }_{\mu }\to {\nu }_e$ transitions by the MINOS experiment based on a $3.14\times {10}^{20}$ protons-on-target exposure in the Fermilab NuMI beam. We observe 35 events in the Far Detector with a background of $27\pm 5(\mathrm{stat})\pm 2(\mathrm{syst})$ events predicted by the measurements in the Near Detector. If interpreted in terms of ${\nu }_{\mu }\to {\nu }_e$ oscillations, this $1.5\sigma$ excess of events is consistent with ${sin}^2(2{\theta }_{13})$ comparable to the CHOOZ limit when $|\Delta m^2|=2.43\times {10}^{-3}{\mathrm{eV}}^2$ and ${sin}^2(2{\theta }_{23})=1.0$ are assumed.

Figure 1

Reconstructed ND energy spectra of the ${\nu }_e$-CC selected backgrounds from NC (dashed) and ${\nu }_{\mu }$-CC (dotted) interactions as obtained from the horn-off method. The shaded histogram shows the beam ${\nu }_e$-CC component from the simulation. The solid histogram corresponds to the total of these three components which are constrained to agree with the data points. The uncertainties shown on the data are statistical and are not visible on this scale; uncertainties on the components are systematic.

Figure 2

Distribution of the ANN selection variable for events in the FD. Black points show data with statistical error bars. The nonshaded histogram shows the background expectation. The shaded region shows the additional ${\nu }_e$-CC events required to explain the observed excess with the oscillation hypothesis.

Figure 3

Reconstructed energy distribution of the ${\nu }_e$-CC selected events in the FD, with the exception of the energy cut. No data events are found below 1 GeV or above 8 GeV, consistent with expectation. Black points show data with statistical error bars. The nonshaded histogram shows the background expectation. The shaded region shows the additional ${\nu }_e$-CC events required to explain the observed excess with the oscillation hypothesis.

Figure 4

Values of ${sin}^2(2{\theta }_{13})$ and ${\delta }_{CP}$ that produce a number of events consistent with the observation for the normal hierarchy (top) and inverted hierarchy (bottom). Black lines show those values that best represent our data. Red (blue) regions show the 90% (68%) C.L. intervals. The parameters $\Delta m^2$ and ${sin}^2(2{\theta }_{23})$ are fixed for comparison to the CHOOZ limit. Electron-neutrino appearance in MINOS is proportional to ${sin}^2({\theta }_{23}){sin}^2(2{\theta }_{13})$. For ${sin}^2(2{\theta }_{23})\ne 1$, the x axis would be rescaled appropriately.