Paper detail

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 ($d_τ$) 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 $d_τ$ as ${\rm Re}(d_τ) = ( -0.62 \pm 0.63 ) \times 10^{-17} ~e{\rm cm}$ and ${\rm Im}(d_τ) = ( -0.40 \pm 0.32 ) \times 10^{-17} ~e{\rm cm}$, 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.