$U(1)$′ mediated decays of heavy sterile neutrinos in MiniBooNE

The MiniBooNE low-energy excess is a long-standing problem which has received further confirmation with a reanalysis using newly collected data, with the anomaly now at the $4.8\sigma$ level. In this paper we propose a novel explanation which advocates a low-energy sector containing $Z^{\prime }$ bosons with GeV-scale masses and sterile neutrinos with masses around 100–500 MeV. We show that this scenario provides excellent spectral agreement with the MiniBooNE low-energy excess in the form of $Z^{\prime }$-mediated neutral current production of heavy sterile states, a fraction of whose subsequent decay to $e^{+}{e}^{-}$ pairs are misidentified as single electronlike electromagnetic showers. Our model inscribes itself in the broad class of models in which sterile neutrinos are charged under new interactions, allowing new couplings to hidden-sector physics. Alongside the electronlike MiniBooNE signature this model also predicts a novel, low-background, signal in LArTPC detectors such as MicroBooNE consisting of two distinguishable electronlike electromagnetic showers originating from a single vertex.

Figure 1

Results of parameter scan in $m_4$ and $m_{Z^{\prime }}$ fitting the shape of our model signal to reconstructed visible energy spectrum (left) and the reconstructed shower angle (right). The energy spectra is sensitivity only to $m_4$, requiring $100\mathrm{MeV}\lesssim m_4\lesssim 200\mathrm{MeV}$ to produce an excess with low enough energy, where as the angular spectrum puts a lower bound on the $Z^{\prime }$ mass below which the signal events are too forward going. The black star shows a representative point, detailed in Fig. (2). The values of the remaining parameters $U_{\mu 4}$, $U_{\tau 4}$, and kinetic mixing $\chi$ are marginalized over.

Figure 2

Our model predictions in relation to the MiniBooNE excess, after subtracting predicted backgrounds, in both reconstructed visible energy (left) and reconstructed shower angle (right), for a 0.14 GeV sterile neutrino and 1.25 GeV $Z^{\prime }$. In a minimal realization, this requires neutrino mixings of $|U_{\mu 4}{|}^2=1.5\times {10}^{-6}$, $|U_{\tau 4}{|}^2=7.8\times {10}^{-4}$ and kinetic mixing ${\chi }^2=5\times {10}^{-6}$, corresponding to a total decay length of 1 m. Excellent agreement is observed in both spectrums. MiniBooNE’s best-fit sterile neutrino oscillation model is shown for comparison (blue dashed line).