A search for the electric dipole moment of the τ-lepton

Abstract Using the ARGUS detector at the e + e − storage ring DORIS II, we have searched for the real and imaginary part of the electric dipole formfactor d τ of the τ lepton in the production of τ pairs at q 2 =100 GeV 2 . This is the first direct measurement of this CP violating formfactor. We applied the method of optimised observables which takes into account all available information on the observed τ decay products. No evidence for CP violation was found, and we derive the following results: Re (d τ )=(1.6±1.9)·10 −16 e cm and Im (d τ )=(−0.2±0.8)·10 −16 e cm , where statistical and systematic errors have been combined.