Search

Precise study of the hadrons production in \bm{e^+e^-}𝐞+𝐞− annihilation at low energies provides important information about interactions of light quarks and spectroscopy of their bound states. These studies are especially important for theoretical calculations of the muon \bm{(g-2)}(𝐠−2) via the Standard Model because of the present difference between theoretical and experimental values exceeds three standard deviations. In this report we will discuss current status and recent results obtained by the CMD-3 experiment and will give a brief look to status of the BaBar and Belle II experiments.

We present preliminary results of a latest measurement at Belle, the branching fraction of rare tau decay, \tau^- \rightarrow \pi^- \nu_{\tau} \ell^+ \ell^-τ−→π−ντℓ+ℓ− (\ell = eℓ=e or \muμ), using a 562 \rm fb^{-1} data set. The branching fraction of \tau^- \rightarrow \pi^- e^+ e^- \nu_\tauτ−→π−e+e−ντ is measured to be \mathbf{ \mathcal{B}(\tau^-\rightarrow \pi^- e^+ e^- \nu_\tau) = (2.11 \pm 0.19 \pm 0.30) \times 10^{-5}}ℬ(𝛕−→𝛑−𝐞+𝐞−𝛎𝛕)=(2.11±0.19±0.30)×10−5, where the first uncertainty is statistical and the second is systematic. An upper limit is set on the branching fraction of \tau^- \rightarrow \pi^- \mu^+ \mu^- \nu_\tauτ−→π−μ+μ−ντ, \mathbf{\mathcal{B}(\tau^-\rightarrow \pi^- \mu^+ \mu^- \nu_\tau) < 1.06 \times 10^{-5}}ℬ(𝛕−→𝛑−𝛍+𝛍−𝛎𝛕)<1.06×10−5, at 90% confidence level.