Precision muon reconstruction in Double Chooz

We describe a muontrack reconstruction algorithm for the reactor anti-neutrino experiment Double Chooz. The Double Choozdetector consists of two optically isolatedvolumes of the liquid scintillator viewed by PMTs, and an Outer Vetoabove these made of crossed scintillator strips. Muonsare reconstructed by their Outer Vetohit positions along with timing information from the other two detector volumes. All muonsare fit under the hypothesis that they are through-going and ultrarelativistic. If the energy depositions suggest that the muonmay 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 muonintersecting the center of the detector, the resolution is View the MathMLsource~40mm in each transverse dimension. High quality muonreconstruction is an important tool for reducing the impact of the cosmogenic isotope background in Double Chooz.