Tau decays into three charged leptons and two neutrinos

We search for the radiative leptonic tau decays τ→ee+e-ντνe and τ→μe+e-ντνμ using 360 fb-1 of data collected by the CLEO-II experiment at the Cornell Electron Storage Ring. We present a first observation of the τ→ee+e-ντνe process. For this channel we measure the branching fraction B(τ→ee+e-ντνe)=(2.7-1.1-0.4-0.3+1.5+0.4+0.1)x10-5. An upper limit is established for the second channel: B(τ→μe+e-ντνμ)<3.2x10-5 at 90% C.L. Both results are consistent with the rates expected from standard model predictions. copyright 1996 The American Physical Society