Search for an anomalous excess of charged-current ${\nu }_e$ interactions without pions in the final state with the MicroBooNE experiment

This article presents a measurement of ${\nu }_e$ interactions without pions in the final state using the MicroBooNE experiment and an investigation into the excess of low-energy electromagnetic events observed by the MiniBooNE Collaboration. The measurement is performed in exclusive channels with ($1e\mathrm{N}p0\pi$) and without ($1e0p0\pi$) visible final-state protons using $6.86\times {10}^{20}$ protons on target of data collected from the Booster Neutrino Beam at Fermilab. Events are reconstructed with the Pandora pattern recognition toolkit and selected using additional topological information from the MicroBooNE liquid argon time projection chamber. Using a goodness-of-fit test, the data are found to be consistent with the predicted number of events with nominal flux and interaction models with a $p$ value of 0.098 in the two channels combined. A model based on the low-energy excess observed in MiniBooNE is introduced to quantify the strength of a possible ${\nu }_e$ excess. The analysis suggests that, if an excess is present, it is not consistent with a scaling of the ${\nu }_e$ contribution to the flux as predicted by the signal model used in the analysis. Combined, the $1e\mathrm{N}p0\pi$ and $1e0p0\pi$ channels do not give a conclusive indication about the tested model, but separately, they both disfavor the low-energy excess model at $>90\%\mathrm{C}.\mathrm{L}.$ The observation in the most sensitive $1e\mathrm{N}p0\pi$ channel is below the prediction and consistent with no excess. In the less sensitive $1e0p0\pi$ channel, the observation at low energy is above the prediction, while overall there is agreement over the full energy spectrum.

Figure 1

Event displays of selected electron neutrino candidate data events. The horizontal axis corresponds to the wire number, which is converted into a distance based on the wire spacing. The vertical axis corresponds to the time of the recorded charge, which is converted to a distance along the TPC drift direction using the drift velocity in the TPC drift direction. The color scale corresponds to the deposited charge. The $1e\mathrm{N}p0\pi$ event shown (a) has a long electron shower and a short proton track attached at the vertex with a large amount of deposited energy. The $1e0p0\pi$ event shown (b) consists of a single electron shower.

Figure 2

Predicted electron neutrino events broken down by true number of visible protons and pions. The final states selected in this analysis ($1e\mathrm{N}p0\pi$ and $1e0p0\pi$) are shown along with all other ${\nu }_e$ interactions ($1e\mathrm{X}p\mathrm{N}\pi$). Pion categorization refers to both charged and neutral pions.

Figure 3

Energy deposited per unit length ($\mathrm{d}E/\mathrm{d}x$) for electron-photon separation. The figure shows $\mathrm{d}E/\mathrm{d}x$ measured in the [0,4] cm range from the shower start point for a combination of events with and without protons. Data from the signal region ($E_{\nu }<0.65\mathrm{GeV}$) are excluded from this validation plot. The contributions to the stacked histogram are composed of charged-current intrinsic ${\nu }_e$ interactions with any number of final-state hadrons in green, ${\nu }_{\mu }$ and neutral-current ${\nu }_e$ interactions that produce one or more ${\pi }^0\mathrm{s}$ in the final-state in light blue, and all other $\nu$ interactions in cyan. Dirt backgrounds are in red and cosmic backgrounds in grayish blue. This categorization is used in all ${\nu }_e$ selection figures.

Figure 4

Selected muon neutrino events compared to simulation split by true interaction categories. The predicted stacked distribution is composed of charged-current ${\nu }_{\mu }$ interactions in cyan, neutral-current backgrounds in dark blue, ${\nu }_e$ interactions in green, and cosmic backgrounds in grayish blue. The shaded band is the systematic uncertainty.

Figure 5

Response of the $1e\mathrm{N}p0\pi$ selection BDT designed to reject events with ${\pi }^0\mathrm{s}$. Background events are predicted to peak at low BDT scores and electron neutrinos at high BDT scores. Events with BDT score above 0.67 are retained as part of the final selection. Gray bands denote the systematic uncertainty on the prediction.

Figure 6

$1e0p0\pi$ selection BDT response. Background events are predicted to peak at low BDT scores and electron neutrino events at high BDT scores. In the final selection, events with BDT scores above 0.72 are retained. Gray bands denote the systematic uncertainty on the prediction.

Figure 7

Area-normalized comparison of data and simulation for the diphoton mass from ${\pi }^0$ candidates. The data and simulation peaks agree to within 1% and fall within 5% of the accepted ${\pi }^0$ mass of $135\mathrm{MeV}/c^2$, demonstrating good energy-scale calibration for EM showers.

Figure 8

Example distributions of BDT input variables in sideband regions. (a) The transverse development angle for events that would pass the $1e0p0\pi$ loose selection but have more than one shower. Most of the events contain ${\pi }^0\mathrm{s}$, and good agreement between data and simulation indicates that this background is well modeled. (b) The shower conversion distance at pre-selection in the $1e\mathrm{N}p0\pi$ low-BDT sideband. Background events with ${\pi }^0\mathrm{s}$ are predicted to typically have a longer conversion distance than those without ${\pi }^0\mathrm{s}$.

Figure 9

Reconstructed neutrino energy for events in $1e\mathrm{N}p0\pi$ (a) and $1e0p0\pi$ (b) selections from the NuMI beam. In the $1e0p0\pi$ sample, both the relatively large ${\overline{\nu }}_e$ content, due to similar ${\nu }_e$ and ${\overline{\nu }}_e$ fluxes, as well as the low non-${\nu }_e$ background content, due to the relatively large ${\nu }_e+{\overline{\nu }}_e$ fraction in the beam, are distinctive features in NuMI that make the spectrum different from the analogous predictions in BNB.

Figure 10

Summary of the impact of systematic uncertainties presented in Sec. 3 for all selected events in the three channels used in the analysis, shown in the 0.15–1.55 GeV energy range that is used for quantitative results. The percent systematic uncertainty by channel is shown in the top panel. The individual unconstrained contributions coming from flux, cross section, and detector plus simulation statistics are shown in blue, orange, and green, respectively. Detector uncertainties account for both Geant4 reinteraction and detector response modeling uncertainties. The total unconstrained systematic uncertainty is in black, and for the $1e\mathrm{N}p0\pi$ and $1e0p0\pi$ selections, a gray area indicates the total constrained uncertainty. The ${\nu }_{\mu }$ uncertainties are not changed by the constraint. In the high-energy region of the $1e0p0\pi$ energy spectrum, where we have very few events, the uncertainties grow to $\mathcal{O}(100\%)$.

Figure 11

Reconstructed neutrino energy for the selected $1e\mathrm{N}p0\pi$ (a) and $1e0p0\pi$ (b) events. The preconstraint number of predicted events is shown broken down by true interaction topology. The constrained prediction using the muon neutrino data is also shown both with (red) and without (black) the eLEE model included. Systematic uncertainties on the constrained prediction are shown as a shaded band. While not shown in the figure, systematic uncertainties on the eLEE model are considered in the analysis. Quantitative results are calculated in 10 bins from 0.15 to 1.55 GeV, shown here starting in the second bin. The lower bound of the first bin is 0.01 GeV.

Figure 12

Selected kinematic distributions for events that pass the $1e\mathrm{N}p0\pi$ selection. Expected events and uncertainties are shown as predicted by the nominal simulation. (a) Electron angle relative to beam direction. (b) Proton kinetic energy.

Figure 13

Selected $1e0p0\pi$ events as a function of electron angle with respect to the beam. Expected events and uncertainties are shown, without the ${\nu }_{\mu }$ constraint applied. (a) All selected events. (b) Low energy selected events from 0.15 to 0.43 GeV.

Figure 14

Results for the simple hypothesis test in the $1e\mathrm{N}p0\pi$ (a), $1e0p0\pi$ (b), and combined (c) channels. The $\mathrm{\Delta }{\chi }^2$ between the intrinsic ${\nu }_e$ model and the eLEE hypotheses is plotted for toy experiments generated with these hypotheses. The $p$ values indicate the fraction of toy experiments with $\mathrm{\Delta }{\chi }^2$ smaller than the observation. The median $p$ value for toy experiments produced assuming the intrinsic ${\nu }_e$ model is also reported.

Figure 15

Results for the signal strength test in the $1e\mathrm{N}p0\pi$ (a), $1e0p0\pi$ (b), and combined (c) channels. The $\mathrm{\Delta }{\chi }^2$ as a function of the signal strength is evaluated with respect to the best-fit signal strength value. The observed confidence interval at a 90% confidence level is indicated with a vertical lines, as well as the expected upper limit in case of no signal.