Updated MiniBooNE neutrino oscillation results with increased data and new background studies

The MiniBooNE experiment at Fermilab reports a total excess of $638.0\pm 52.1(\mathrm{stat}.)\pm 122.2(\mathrm{syst}.)$ electronlike events from a data sample corresponding to $18.75\times {10}^{20}$ protons-on-target in neutrino mode, which is a 46% increase in the data sample with respect to previously published results and $11.27\times {10}^{20}$ protons-on-target in antineutrino mode. The overall significance of the excess, $4.8\sigma$, is limited by systematic uncertainties, assumed to be Gaussian, as the statistical significance of the excess is $12.2\sigma$. The additional statistics allow several studies to address questions on the source of the excess. First, we provide two-dimensional plots in visible energy and the cosine of the angle of the outgoing lepton, which can provide valuable input to models for the event excess. Second, we test whether the excess may arise from photons that enter the detector from external events or photons exiting the detector from ${\pi }^0$ decays in two model independent ways. Beam timing information shows that almost all of the excess is in time with neutrinos that interact in the detector. The radius distribution shows that the excess is distributed throughout the volume, while tighter cuts on the fiducial volume increase the significance of the excess. The data likelihood ratio disfavors models that explain the event excess due to entering or exiting photons.

Figure 1

The Michel electron energy distribution for the first, second, and third running periods in neutrino mode. The events are normalized to the first running period. The bottom plot shows ratios of the second and third running periods to the first running period.

Figure 2

The MiniBooNE neutrino mode electron-muon particle identification distributions, corresponding to the total $18.75\times {10}^{20}$ POT data in the $200<E_{\nu }^{\mathrm{QE}}<1250\mathrm{MeV}$ energy range, for ${\nu }_e$ CCQE data (points with statistical errors) and background (colored histogram). The dashed histogram shows the best fit to the neutrino-mode data assuming two-neutrino oscillations.

Figure 3

The MiniBooNE neutrino mode electron-pion particle identification distributions, corresponding to the total $18.75\times {10}^{20}$ POT data in the $200<E_{\nu }^{\mathrm{QE}}<1250\mathrm{MeV}$ energy range, for ${\nu }_e$ CCQE data (points with statistical errors) and background (colored histogram). The dashed histogram shows the best fit to the neutrino-mode data assuming two-neutrino oscillations.

Figure 4

The MiniBooNE neutrino mode two-ring invariant-mass particle identification distributions, corresponding to the total $18.75\times {10}^{20}$ POT data in the $200<E_{\nu }^{\mathrm{QE}}<1250\mathrm{MeV}$ energy range, for ${\nu }_e$ CCQE data (points with statistical errors) and background (colored histogram). The dashed histogram shows the best fit to the neutrino-mode data assuming two-neutrino oscillations.

Figure 5

The ${\nu }_{\mu }$ CCQE muon visible energy distribution for the first, second, and third running periods in neutrino mode. The events are normalized to the first running period. The bottom plot shows ratios of the second and third running periods to the first running period.

Figure 6

The NC ${\pi }^0$ mass distribution for the first, second, and third running periods in neutrino mode. The events are normalized to the first running period. The bottom plot shows ratios of the second and third running periods to the first running period.

Figure 7

The MiniBooNE neutrino mode visible energy distributions, corresponding to the total $18.75\times {10}^{20}$ POT data in the $200<E_{\nu }^{\mathrm{QE}}<1250\mathrm{MeV}$ energy range, for ${\nu }_e$ CCQE data (points with statistical errors) and background (colored histogram). The dashed histogram shows the best fit to the neutrino-mode data assuming two-neutrino oscillations.

Figure 8

The MiniBooNE neutrino mode $\cos \theta$ distributions, corresponding to the total $18.75\times {10}^{20}$ POT data in the $200<E_{\nu }^{\mathrm{QE}}<1250\mathrm{MeV}$ energy range, for ${\nu }_e$ CCQE data (points with statistical errors) and background (colored histogram). The dashed histogram shows the best fit to the neutrino-mode data assuming two-neutrino oscillations.

Figure 9

The MiniBooNE neutrino mode $E_{\nu }^{\mathrm{QE}}$ distributions, corresponding to the total $18.75\times {10}^{20}$ POT data in the $200<E_{\nu }^{\mathrm{QE}}<3000\mathrm{MeV}$ energy range, for ${\nu }_e$ CCQE data (points with statistical errors) and predicted backgrounds (colored histograms). The constrained background is shown as additional points with systematic error bars. The dashed histogram shows the best fit to the neutrino-mode data assuming two-neutrino oscillations. The last bin is for the energy interval from 1500–3000 MeV.

Figure 10

The total event excesses in neutrino mode as a function of $E_{\nu }^{\mathrm{QE}}$ for the first, second, and third running periods. Error bars include only statistical uncertainties.

Figure 11

The total numbers of data events in neutrino mode as functions of reconstructed visible energy and $\cos \theta$. There are 20 columns of visible energy from 150 to 1250 MeV and 20 rows of $\cos \theta$ from $-1$ to 1.

Figure 12

The total numbers of background events in neutrino mode as functions of reconstructed visible energy and $\cos \theta$. There are 20 columns of visible energy from 150 to 1250 MeV and 20 rows of $\cos \theta$ from $-1$ to 1.

Figure 13

The total numbers of excess events in neutrino mode as functions of reconstructed visible energy and $\cos \theta$. There are 20 columns of visible energy from 150 to 1250 MeV and 20 rows of $\cos \theta$ from $-1$ to 1.

Figure 14

The $\cos \theta$ distribution of data events (points with error bars) and background events (colored histogram) in neutrino mode for the 20 different visible energy bins from 150 to 1250 MeV.

Figure 15

The $\cos \theta$ distributions from 0.9 to 1 of data events (points with statistical errors) and background events (histogram) in neutrino mode for ten different visible energy bins from 150 to 1250 MeV. Neutrino-electron elastic scattering events are shown as the hatched region in the “others” category.

Figure 16

The MiniBooNE neutrino mode $\cos \theta$ distribution for $\cos \theta >0.9$, corresponding to the total $18.75\times {10}^{20}$ POT neutrino data in the visible energy range from 150 to 1250 MeV, for ${\nu }_e$ CCQE data (points with statistical errors) and predicted backgrounds (colored histograms). Neutrino-electron elastic scattering events are shown as the hatched region in the “others” category.

Figure 17

The MiniBooNE neutrino mode $E_{\nu }^{\mathrm{QE}}$ distributions, corresponding to the total $18.75\times {10}^{20}$ POT neutrino data in the $150<E_{\nu }^{\mathrm{QE}}<1250\mathrm{MeV}$ energy range, for ${\nu }_e$ CCQE data (points with statistical errors) and predicted backgrounds (colored histograms). The dashed histogram shows the best fit to the neutrino-mode data assuming two-neutrino oscillations.

Figure 18

The MiniBooNE neutrino mode $\cos \theta$ distributions, corresponding to the total $18.75\times {10}^{20}$ POT neutrino data in the $150<E_{\nu }^{\mathrm{QE}}<1250\mathrm{MeV}$ energy range, for ${\nu }_e$ CCQE data (points with statistical errors) and predicted backgrounds (colored histograms). The dashed histogram shows the best fit to the neutrino-mode data assuming two-neutrino oscillations.

Figure 19

The total event excess in neutrino mode, corresponding to $18.75\times {10}^{20}$ POT in the $150<E_{\nu }^{\mathrm{QE}}<3000\mathrm{MeV}$ energy range. The solid curve shows the best fit to the neutrino-mode and antineutrino-mode data in the $200<E_{\nu }^{\mathrm{QE}}<1250\mathrm{MeV}$ energy range assuming two-neutrino oscillations, while the dashed curves show selected points on the 1 sigma contour. Also shown is the 1-sigma allowed band. The outer error bars include statistical plus constrained systematic uncertainties, while the inner error bars show the statistical uncertainties. The lowest energy data point shows only the unconstrained background statistical uncertainty.

Figure 20

MiniBooNE allowed regions for combined neutrino mode ($18.75\times {10}^{20}$ POT) and antineutrino mode ($11.27\times {10}^{20}$ POT) data sets for events with $200<E_{\nu }^{\mathrm{QE}}<3000\mathrm{MeV}$ within a two-neutrino oscillation model. The shaded areas show the 90% and 99% C.L. LSND ${\overline{\nu }}_{\mu }\to {\overline{\nu }}_e$ allowed regions. The black point shows the MiniBooNE best fit point. Also shown are 90% C.L. limits from the KARMEN [27] and OPERA [28] experiments.

Figure 21

A comparison between the $L/E_{\nu }^{\mathrm{QE}}$ distributions for the MiniBooNE data excesses in neutrino mode ($18.75\times {10}^{20}$ POT) and antineutrino mode ($11.27\times {10}^{20}$ POT) to the $L/E$ distribution from LSND [1]. The error bars show statistical uncertainties only. The curves show fits to the MiniBooNE data, assuming two-neutrino oscillations, while the shaded area is the MiniBooNE $1\sigma$ allowed band. The best-fit curve corresponds to $({\sin }^{2}2\theta ,\mathrm{\Delta }{m}^2)=(0.807,0.043{\mathrm{eV}}^2)$, while the dashed curve corresponds to a $1\sigma$ fit point at $({\sin }^{2}2\theta ,\mathrm{\Delta }{m}^2)=(0.01,0.4{\mathrm{eV}}^2)$.

Figure 22

The MiniBooNE radial vertex distribution, corresponding to the total $18.75\times {10}^{20}$ POT data in neutrino mode in the $200<E_{\nu }^{\mathrm{QE}}<1250\mathrm{MeV}$ energy range, for ${\nu }_e$ CCQE data (points with statistical errors) and background (histogram). The dashed histogram shows the best fit to the neutrino-mode data assuming two-neutrino oscillations.

Figure 23

The excess event radial distributions in neutrino mode with only statistical errors in the $200<E_{\nu }^{\mathrm{QE}}<1250\mathrm{MeV}$ energy range, where different processes are normalized to explain the event excess. The different processes are the following: (a) $\mathrm{\Delta }\to N\gamma$; (b) external events; (c) ${\nu }_e$ and ${\overline{\nu }}_e$ from $K_L^0$ decay; (d) ${\nu }_e$ and ${\overline{\nu }}_e$ from $K^{\pm }$ decay; (e) ${\nu }_e$ and ${\overline{\nu }}_e$ from ${\mu }^{\pm }$ decay; (f) other ${\nu }_e$ and ${\overline{\nu }}_e$; (g) NC ${\pi }^0$; (h) best fit oscillations.

Figure 24

The bunch timing for data events in neutrino mode compared to the expected background in the $200<E_{\nu }^{\mathrm{QE}}<1250\mathrm{MeV}$ energy range. Almost all of the excess data events occur, as expected from neutrino events in the detector, within the first 8 ns of the bunch timing. This data sample uses events collected with the new fiber timing system and represents about 40% of the entire neutrino mode sample.

Figure 25

The MiniBooNE neutrino mode $E_{\nu }^{\mathrm{QE}}$ distributions, corresponding to the total $18.75\times {10}^{20}$ POT data in neutrino mode in the $200<E_{\nu }^{\mathrm{QE}}<1250\mathrm{MeV}$ energy range, for ${\nu }_e$ CCQE data (points with statistical errors) and background (histogram) with radius less than 4 m. The dashed histogram shows the best fit to the neutrino-mode data assuming two-neutrino oscillations.

Figure 26

The MiniBooNE neutrino mode $\cos \theta$ distributions, corresponding to the total $18.75\times {10}^{20}$ POT data in neutrino mode in the $200<E_{\nu }^{\mathrm{QE}}<1250\mathrm{MeV}$ energy range, for ${\nu }_e$ CCQE data (points with statistical errors) and background (histogram) with radius less than 4 m. The dashed histogram shows the best fit to the neutrino-mode data assuming two-neutrino oscillations.