  We describe a muon track reconstruction algorithm for the reactor
anti-neutrino experiment Double Chooz. The Double Chooz detector consists of
two optically isolated volumes of liquid scintillator viewed by PMTs, and an
Outer Veto above these made of crossed scintillator strips. Muons are
reconstructed by their Outer Veto hit positions along with timing information
from the other two detector volumes. All muons are fit under the hypothesis
that they are through-going and ultrarelativistic. If the energy depositions
suggest that the muon may have stopped, the reconstruction fits also for this
hypothesis and chooses between the two via the relative goodness-of-fit. In the
ideal case of a through-going muon intersecting the center of the detector, the
resolution is ~40 mm in each transverse dimension. High quality muon
reconstruction is an important tool for reducing the impact of the cosmogenic
isotope background in Double Chooz.
