Evidence for new interference phenomena in the decay D+ to K- pi+ mu+ nu

Using a large sample of charm semileptonic decays collected by the FOCUS photoproduction experiment at Fermilab, we present evidence for a small, even spin K- \pi+ amplitude that interferes with the dominant K*0 component in the K-pi+ mu- nu final state. Although this interference significantly distorts the decay angular distributions, the new amplitude creates only a very small distortion to the observed kaon pion mass distribution when integrated over the other kinematic variables describing the decay. Our data can be described by K*0 interference with either a constant amplitude or broad spin zero resonance.