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(${}_4$S) 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(d${}_{\tau }$) = (−0.62 ± 0.63) × 10${}^{-17}$ ecm and Im(d${}_{\tau }$) = (−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.