Overview of tau lepton physics at a super tau-charm facility

TL;DR

This paper surveys tau lepton physics in the context of a proposed super tau-charm factory ($STCF$). It traces the tau's discovery and current precision measurements, and outlines how the STCF's energy reach and luminosity can advance mass, lifetime, branching fractions, LFU/LFV, EDM, and CP-violation studies, as well as hadronic spectral functions relevant to muon $g-2$ and $\alpha_s$ determinations. The work highlights opportunities in measuring $m_\tau$, $\tau_\tau$, $B_e$, and searching for LFV decays like $\tau \to \mu\gamma$, with projected sensitivities that could surpass current limits. It also notes the role of spectral functions in precise SM tests and as inputs to hadronic vacuum polarization predictions, and emphasizes the need for feasibility studies of accelerator and detector optimization.

Abstract

An overview of tau lepton physics is presented, using the tau lepton discovery and its precision measurements as examples to illustrate the importance of the energy region to be covered by the super tau-charm factory. By presenting the current measurement status of the major physics topics, the emphasize is put on pointing out a few open issues and possibilities for the super tau-charm factory.

Figures (7)

• Figure 1: Tau pair production cross section as a function of center-of-mass energy. Figure taken from Ref. Perl:1991gd.
• Figure 2: Measured $\tau_\tau$, ${\@fontswitch{}{\mathcal{}} B}_e$ in comparison with the expected value using the measured $m_\tau$. These measured values correspond to averaged one from HFLAV HeavyFlavorAveragingGroupHFLAV:2024ctg. The size of the error band is due to the uncertainty of $m_\tau$.
• Figure 3: Graphs for tau lepton-flavor-violating decays from the SM and new physics contributions.
• Figure 4: Tau lepton-flavor-violating branching fraction upper limits and combinations. Figure taken from Ref. HeavyFlavorAveragingGroupHFLAV:2024ctg.
• Figure 5: Measured $V+A$ and $V-A$ spectral functions from ALEPH. Figures taken from Ref. Davier:2013sfa.