  A search for millicharged particles, a simple extension of the standard
model, has been performed with the ArgoNeuT detector exposed to the Neutrinos
at the Main Injector beam at Fermilab. The ArgoNeuT Liquid Argon Time
Projection Chamber detector enables a search for millicharged particles through
the detection of visible electron recoils. We search for an event signature
with two soft hits (MeV-scale energy depositions) aligned with the upstream
target. For an exposure of the detector of $1.0$ $\times$ $10^{20}$ protons on
target, one candidate event has been observed, compatible with the expected
background. This search is sensitive to millicharged particles with charges
between $10^{-3}e$ and $10^{-1}e$ and with masses in the range from $0.1$ GeV
to $3$ GeV. This measurement provides leading constraints on millicharged
particles in this large unexplored parameter space region.
