An improved search for the electric dipole moment of the τ lepton

We report a measurement of the electric dipole moment of the τ lepton (dτ) using an 833 fb−1 data sample collected near the Υ(4S) resonance, with the Belle detector at the KEKB asymmetric-energy e+e− collider. Using an optimal observable method, we obtain the real and imaginary parts of dτ as Re(dτ) = (−0.62 ± 0.63) × 10−17 ecm and Im(dτ) = (−0.40 ± 0.32) × 10−17 ecm, respectively. These results are consistent with null electric dipole moment at the present level of experimental sensitivity and improve the sensitivity by about a factor of three.