Shedding light on the MiniBooNE excess with searches at the LHC

The origin of the excess of low-energy events observed by the MiniBooNE experiment remains a mystery, despite exhaustive investigations of backgrounds and a series of null measurements from complementary experiments. One intriguing explanation is the production of beyond-the-Standard-Model particles that could mimic the experimental signature of additional ${\nu }_e$ appearance seen in MiniBooNE. In one proposed mechanism, muon neutrinos up-scatter to produce a new “dark neutrino” state that decays by emitting highly collimated electron-positron pairs. We propose high-energy neutrinos produced from $W$ boson decays at the Large Hadron Collider as an ideal laboratory to study such models. Simple searches for a low-mass, boosted dilepton resonance produced in association with a high-$p_{\mathrm{T}}$ muon from the $W$ decay with run 2 data would already provide unique sensitivity to a range of dark neutrino scenarios, with prompt and displaced searches providing complementarity. Looking farther ahead, we show how the unprecedented sample of $W$ boson decays anticipated at the HL-LHC, together with improved lepton acceptance would explore much of the parameter space most compatible with the MiniBooNE excess.

Figure 1

Representative diagrams are shown for the production of $e^{+}{e}^{-}$ pairs from $Z_D$ decays in $W^{-}\to {\mu }^{-}{\overline{\nu }}_{\mu }$ events, produced either through the mixing of neutral leptons (left) or $U(1)$ gauge forces (center). At right the dominant SM background process is shown, in which the muon radiates an $e^{+}{e}^{-}$ pair via virtual photon emission.

Figure 2

The dielectron mass resolution is shown for electrons for the optimistic and pessimistic scenarios described in the text. Single-electron resolutions are applied to simulated events in the dark neutrino signal model for a 10 GeV $N_D$ mass. The quoted resolution is computed as the smallest interval containing 68% of the smeared mass values.

Figure 3

The expected distribution of BSM signal and background processes are shown for events passing the pre-selection criteria, normalized to $150{\mathrm{fb}}^{-1}$ of collected data. SM background contributions are shown cumulatively for $W$, $Z$, and top processes, while realizations of the signal model for different $Z_D$ and $N_D$ masses are overlaid, fixing $|U_{\mu 4}{|}^2={10}^{-4}$. The electron channel is shown for events with lepton $p_{\mathrm{T}}>1\mathrm{GeV}$. Overflow values falling below (above) the x-axis limits are included in the first (last) bin.

Figure 4

The expected distribution of BSM signal and background processes are shown for events passing the signal region selection, normalized to $150{\mathrm{fb}}^{-1}$ of collected data. SM background contributions are shown cumulatively for $W$, $Z$, and top processes, while realizations of the signal model for different $Z_D$ and $N_D$ masses are overlaid, fixing $|U_{\mu 4}{|}^2={10}^{-6}$. The electron channel is shown for events with lepton $p_{\mathrm{T}}>1\mathrm{GeV}$. Overflow values falling below (above) the $x$-axis limits are included in the first (last) bin.

Figure 5

At left, the distribution of displacements is shown for various combinations of signal model parameters, normalized to unity. The subset of events passing the full event selection is also shown for comparison. At right, the same distributions are shown for events passing the full event selection with variations of the kinetic mixing parameter $\epsilon$. Events falling below or above of the displayed range of displacements are added to the first or last bin of displacement, respectively.

Figure 6

The ranges of neutrino mixing parameters expected to be excluded by LHC experiments during run 2 and the HL-LHC are shown versus $N_D$ mass for a fixed $Z_D$ mass of 30 MeV. Upper limits on $U_{\mu 4}$ are shown for both the prompt and displaced search strategies, with uncertainty bands described in the text. Closed contours correspond to the parameters favored by Ref. [10] at the 1, 2, 3, 4 and 5-sigma confidence level and shaded gray regions correspond to prior constraints, described in the text.

Figure 7

Expected upper limits on the neutrino mixing parameter $U_{\mu 4}$ are shown as a function of the $Z_D$ mass under the assumption that $m_{N_D}=3m_{Z_D}$. The results of the prompt and displaced analysis are separately presented, assuming both the run 2 and HL-LHC datasets.

Figure 8

Expected upper limits on the neutrino mixing parameter $U_{\mu 4}$ are shown as a function of the $Z_D$ mass for several assumptions on the kinetic mixing parameter. Comparisons are made for the run 2 (top row) and HL-LHC datasets (bottom row), and for the $m_{Z_D}=30\mathrm{MeV}$ (left column) and $m_{N_D}=3m_{Z_D}$ (right column) assumptions. In each case, the results of both the prompt and displaced searches are shown, assuming values of $\alpha {\epsilon }^2=\times {10}^{-9}$, $\alpha {\epsilon }^2=\times {10}^{-10}$, and $\alpha {\epsilon }^2=\times {10}^{-11}$.

Figure 9

Expected upper limits on the neutrino mixing parameter $U_{\mu 4}$ are shown as a function of the $Z_D$ mass for several assumptions about the classes of potentially reconstructable leptons. Comparisons are made for the run 2 (top row) and HL-LHC datasets (bottom row), and for the $m_{Z_D}=30\mathrm{MeV}$ (left column) and $m_{N_D}=3m_{Z_D}$ (right column) assumptions.