Probing the electromagnetic dipole moment of the $\tau$ lepton in the $e^+e^- \to \gamma^*/Z \to \tau^+ \tau^-$ reaction

Abstract

High-precision measurements of the lepton's electromagnetic dipole moments provide a powerful probe for testing the Standard Model. A non-zero value of the $\tau$ lepton's electric(weak) dipole moment ($d_{\tau}^{\gamma}$, $d_{\tau}^{Z}$) would serve as a smoking gun of the new physics. On the one hand, current and future high energy colliders offer an ideal environment for such measurements. On the other hand, it is essential to investigate the optimal measurement method for extracting not only $d_{\tau}^{\gamma}$ and $d_{\tau}^{Z}$, but also the anomalous magnetic dipole moment ($a_{\tau}$). In this work, we analyze the precision of observables, particularly optimal observables, for determining these physics quantities through the process $e^+e^- \to \gamma^*/Z \to \tau^+ \tau^-$ with $\tau$ leptons undergoing (semi-)leptonic and hadronic decays at the centre-of-mass energies at the Z pole mass and significantly below $m_Z$. By considering the full kinematic information of the decay products, we find that the sensitivities to Im$d_{\tau}$ and Re$d_{\tau}$ can reach $10^{-21}$ $ecm$ at CEPC $\sqrt s = m_Z$, compared to $10^{-20}$ $ecm$ at Belle-II $\Upsilon(4S)$ resonance and $10^{-17}$ $ecm$ at BEPCII $\psi(2S)$ resonance. For $a_{\tau}$, we find that Im$a_{\tau}$ and Re$a_{\tau}$ with a precision of $10^{-5}$ at Belle-II $\sqrt s=10.58$ GeV can be attained.