  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, $Y=\epsilon^2\alpha_D(m_\chi/m_V)^4 \lesssim10^{-8}$,
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.
