Tau lepton experiment at BES detector

Abstract

(1) A data-driven energy scan in the immediate vicinity of τ pair production threshold has been performed using the Beijing Spectrometer (BES) at the Beijing Electron-Positron Collider (BEPC). Approximately 5 pb −1 of data, distributed over twelve scan points, have been collected. An initial τ lepton mass value, obtained using only the eμ final state, has been published. In this paper, the final BES result on the mass measurement, based on the combined data samples of ee , eμ , eπ , eK , μμ , μπ , μK , ππ and πK events, is presented. A maximum likelihood fit yields the value m τ = 1776.96 −0.19−0.16 +0.18+0.20 MeV. (2) Using the data collected at the center-of-mass energy 4.03 GeV, the decay τ − → ν τ + 3 charged pseudoscalars has been investigated. The branching ratios are determined to be Br( τ − → α 1 ν τ → ϱ 0 π ± ν τ , ϱ 0 → π + π − )=(7.3±0.5)%, Br( τ − → K * ± ν τ → K s 0 π ± ν τ , K s 0 → π + π − ) = (6.0±1.5) × 10 −3 . The decay channels with kaon have also been studied. (3) The braching ratio of ψ (2 s ) → τ + τ − has been determined as (3.69±0.71±0.66) × 10 −3 in the first time.