Dark Matter Search in a Proton Beam Dump with MiniBooNE

The MiniBooNE-DM Collaboration searched for vector-boson mediated production of dark matter using the Fermilab 8-GeV Booster proton beam in a dedicated run with $1.86\times 10^{20}$ protons delivered to a steel beam dump. The MiniBooNE detector, 490 m downstream, is sensitive to dark matter via elastic scattering with nucleons in the detector mineral oil. Analysis methods developed for previous MiniBooNE scattering results were employed, and several constraining data sets were simultaneously analyzed to minimize systematic errors from neutrino flux and interaction rates. No excess of events over background was observed, leading to a 90% confidence limit on the dark matter cross section parameter, $Y={\epsilon }^2{\alpha }_D(m_{\chi }/m_V{)}^4\lesssim {10}^{-8}$, for ${\alpha }_D=0.5$ and for dark matter masses of $0.01<m_{\chi }<0.3\mathrm{GeV}$ in a vector portal model of dark matter. This is the best limit from a dedicated proton beam dump search in this mass and coupling range and extends below the mass range of direct dark matter searches. These results demonstrate a novel and powerful approach to dark matter searches with beam dump experiments.

Figure 1

Schematic illustration of this DM search using the Fermilab BNB in off-target mode together with the MiniBooNE detector. The proton beam is steered above the beryllium target in off-target mode, lowering the neutrino flux.

Figure 2

DM-production channels relevant for this search with an 8-GeV proton beam incident on a steel target.

Figure 3

DM interactions with nucleons in the detector.

Figure 4

Predicted flux (top) in off-target mode and the flux ratio for off-target to $\nu$ mode (bottom) as a function of neutrino energy for each neutrino species.

Figure 5

Reconstructed nucleon kinetic energy distribution for DM-candidate events with the experimental data shown as circles with statistical error bars. The predicted backgrounds are shown as lines and the results from a background-only fit to the combined data set are shown as triangles with error boxes. The bottom plot shows the data and unconstrained background-only prediction together with example DM signals as a ratio to the background-only fit. The example signals are the 90% confidence-limit solutions at the best-fit point (DM1, $m_V=10\mathrm{MeV}$, $m_{\chi }=1\mathrm{MeV}$, ${\epsilon }^4{\alpha }_D=8.1\times {10}^{-14}$) and the most sensitive point (DM2, $m_V=769\mathrm{MeV}$, $m_{\chi }=381\mathrm{MeV}$, ${\epsilon }^4{\alpha }_D=1.3\times {10}^{-14}$).

Figure 6

The ${\epsilon }^4{\alpha }_D$ 90% confidence limits for $0.01<m_V<1\mathrm{GeV}$ and $m_V>2m_{\chi }$ using the vector portal DM model.

Figure 7

Confidence limits and sensitivities with $1,2\sigma$ errors resulting from this analysis compared to other experimental results [4, 11, 12, 31, 32, 33, 34, 35, 36, 37]. Limits from experiments that assume DM coupling to quarks or nucleons, including this result, are shown as solid lines, while those that require DM coupling to electrons are shown as dot-dashed lines. The favored parameters for this model to account for the observed relic DM density [4] are shown as the lowest solid line.