Using data from the FOCUS (E831) experiment at Fermilab, we present new measurements for the Cabibbo-suppressed decay mode D0→π-π+π-π+. We measure the branching ratio Γ(D0→π+π-π+π-)/ Γ(D0→K-π+π-π+)=0.0914±0.0018±0.0022. An amplitude analysis has been performed, a first for this channel, in order to determine the resonant substructure of this decay mode. The dominant component is the decay D0→a1(1260)+π-, accounting for 60% of the decay rate. The second most dominant contribution comes from the decay D0→ρ(770) 0ρ(770)0, with a fraction of 25%. We also study the a1(1260) line shape and resonant substructure. Using the helicity formalism for the angular distribution of the decay D0→ρ(770)0ρ(770)0, we measure a longitudinal polarization of PL=(71±4±2)%. © 2007 The American Physical Society.
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