First Search for Dark Sector <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:msup> <mml:mrow> <mml:mi>e</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> </mml:mrow> </mml:msup> <mml:msup> <mml:mrow> <mml:m

We present MicroBooNE’s first search for dark sector <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:msup> <a:mi>e</a:mi> <a:mo>+</a:mo> </a:msup> <a:msup> <a:mi>e</a:mi> <a:mo>−</a:mo> </a:msup> </a:math> explanations of the long-standing MiniBooNE anomaly. The MiniBooNE anomaly has garnered significant attention over the past 20 years including previous MicroBooNE investigations into both anomalous electron and photon excesses, but its origin still remains unclear. In this Letter, we provide the first direct test of dark sector models in which dark neutrinos, produced through neutrino-induced scattering, decay into missing energy and visible <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:msup> <c:mi>e</c:mi> <c:mo>+</c:mo> </c:msup> <c:msup> <c:mi>e</c:mi> <c:mo>−</c:mo> </c:msup> </c:math> pairs comprising the MiniBooNE anomaly. Many such models have recently gained traction as a viable solution to the anomaly while evading past bounds. Using an exposure of <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:mn>6.87</e:mn> <e:mo>×</e:mo> <e:msup> <e:mn>10</e:mn> <e:mn>20</e:mn> </e:msup> </e:math> protons-on-target in the Booster Neutrino Beam, we implement a selection targeting forward-going, coherently produced <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"> <g:msup> <g:mi>e</g:mi> <g:mo>+</g:mo> </g:msup> <g:msup> <g:mi>e</g:mi> <g:mo>−</g:mo> </g:msup> </g:math> events. After unblinding, we observe 95 events, which we compare with the constrained background-only prediction of <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"> <i:mn>69.7</i:mn> <i:mo>±</i:mo> <i:mn>17.3</i:mn> </i:math> . This analysis sets the world’s first direct limits on these dark sector models and, at the 95% confidence level, excludes the entirety of the single dark neutrino and majority of the dual dark neutrino, parameter space that is viable as a solution to the MiniBooNE anomaly.