  Neutrino experiments lie at the edge of the intensity frontier and therefore
can be exploited to search for new light particles weakly coupled to the
visible sector. In this work we derive new constraints on axion-like particles
(ALPs) using data from the MicroBooNE experiment, from a search for $e^+ e^-$
pairs pointing in the direction of the NuMI absorber. In particular, we
consider the addition of higher-dimensional effective operators coupling the
ALP to the electroweak gauge bosons. These would induce $K\to \pi a$ from kaon
decay at rest in the NuMI absorber, as well as ALP decays into pairs of leptons
or photons. We discuss in detail and compare various results obtained for the
decay width $K\rightarrow \pi a$ in previous literature. For the operator
involving the Higgs, MicroBooNE already sets competitive bounds (comparable to
those of NA62) for ALP masses between 100 and 200 MeV. We also compute the
expected sensitivities from the full NuMI dataset recorded at MicroBooNE. Our
results show that a search for a $a\to\gamma\gamma$ signal may be able to
improve over current constraints from beam-dump experiments on the operator
involving the ALP coupling to the $W$.
