Narrow Results

(τ⁺τ^-) is an atom of simple hydrogenlike structure similar to positronium (e^-e⁺) and (μ⁺μ^-). In this paper energy levels and decay widths of different decay channels of (τ⁺τ^-) are given. Cross-section of production of this atomic system in e^-e⁺ annihilation taking into account radiative corrections is calculated. According to our estimates 31 (τ⁺τ^-) atoms are produced at VEPP-4M and 943 (τ⁺τ^-) atoms may be produced at BEPCII under the present experimental conditions.

Using the massive helicity formalism, we calculate the five-body average square amplitude of the decays ${\tau }^{-}\to {\ell }_i^{-}{\ell }_i^{+}{\ell }_j^{-}{\bar{\nu }}_j{\nu }_{\tau }$$(\ell =e,\mu )$ within the Standard Model (SM), we then introduce a dimension-five effective vertex ${\mathrm{\Gamma }}^{\tau \tau \gamma }$ in order to determine the feasibility of imposing limits on the tau anomalous magnetic dipole moment $(a_{\tau })$ via the current or future experimental measurements of the branching ratio for the decay ${\tau }^{-}\to e^{-}{e}^{+}{e}^{-}{\bar{\nu }}_e{\nu }_{\tau }$.

In this paper, we present the major results obtained in $\tau$ physics by the Belle and Belle II experiments. For the Belle II experiment, we also discuss prospects for improved measurements of the $\tau$ lepton properties, new results in Michel parameters determination, and searches for CP and lepton flavor violation in $\tau$ decays.