An improved search for the electric dipole moment of the $\tau$ lepton

TL;DR

This paper reports an improved measurement of the tau lepton's electric dipole moment using Belle detector data, setting more stringent limits and finding results consistent with zero.

Contribution

It introduces an enhanced analysis method that improves sensitivity to the tau EDM by a factor of three compared to previous measurements.

Findings

Measured Re(d_tau) = (-0.62 ± 0.63) × 10^{-17} e·cm

Measured Im(d_tau) = (-0.40 ± 0.32) × 10^{-17} e·cm

Results are consistent with a null electric dipole moment.

Abstract

We report a measurement of the electric dipole moment of the $\tau$ lepton ($d_\tau$) using an 833~fb$^{-1}$ data sample collected near the $\Upsilon(4S)$ 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 $d_\tau$ as ${\rm Re}(d_\tau) = ( -0.62 \pm 0.63 ) \times 10^{-17} ~e{\rm cm}$ and ${\rm Im}(d_\tau) = ( -0.40 \pm 0.32 ) \times 10^{-17} ~e{\rm cm}$, 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.