Probing the tau electric dipole moment at BEPC-II collider energies

TL;DR

This paper evaluates the potential to detect a nonzero tau electric dipole moment at BEPC-II energies by analyzing CP-odd observables in tau pair production and decay, estimating sensitivities achievable with realistic luminosities.

Contribution

It introduces a detailed method to measure the tau EDM form factor at specific collider energies using CP-odd observables and realistic experimental assumptions.

Findings

Achieves sensitivity to Re d_tau of 4.5e-18 ecm at 4.18 GeV

Achieves sensitivity to Im d_tau of 2.2e-18 ecm at 4.18 GeV

Provides a framework for tau EDM measurement at BEPC-II energies.

Abstract

We investigate the prospects of the search for a nonzero $\tau$ EDM form factor $d_\tau(s)$ in $\tau$ pair production by $e^+ e^-$ collisions at BEPC-II collider energies. We compute the expectation values and covariances of simple and optimal $CP$-odd observables for $\tau$-pair production at $\sqrt{s}=4.18$ GeV and $4.95$ GeV with subsequent decays of $\tau^\pm$ into major leptonic or semihadronic modes. For the $\tau$ decays to two pions and three charged pions we take the full kinematic information of the hadronic system into account. Applying cuts and using realistic assumptions on the eventually attainable integrated luminosities at these energies, ${\cal L}(4.18) = 3\times 10^4~{\rm pb}^{-1}$ and ${\cal L}(4.95) = 10^4~{\rm pb}^{-1}$, respectively, we find the following. By taking into account purely semihadronic and semihadronic-leptonic $\tau^+\tau^-$ decays one can achieve with optimal $CP$-odd observables the 1 s.d. sensitivities $\delta {\rm Re} d_\tau = 4.5\times 10^{-18}$ ecm ($5.3\times 10^{-18}$ ecm) and $\delta {\rm Im} d_\tau = 2.2 \times 10^{-18}$ ecm ($2.7\times 10^{-18}$ ecm) at $\sqrt{s}=4.18$ GeV ($4.95$ GeV).