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