Measurement of the mass of the tau lepton.

A data-driven energy scan in the immediate vicinity of r pair production threshold has been performed using the Beijing Spectrometer at the Beijing Electron-Positron Collider. Approximately 5 pb-l of data, distributed over twelve scan points, have been collected. A previous mass value for the T lepton, obtained using only the ep final state, has been published. In this paper, the final BES result on the mass measurement is presented. The analysis is based on the combined data from the ee, ep, eh, pp, ph, and hh final states, where h denotes a charged r or Ir’. A maximum likelihood fit to the T pair production cross section data yields the value m7 = 1776.96 ‘“,:fy ti:f”, MeV. Submitted to Physical Review D *Work supported in part by the National Natural Science Foundation of China under Contract No. 19290400 and the Chinese Academy of Sciences under contract No. KJ85 (IHEP); by the Department of Energy under Contract Nos. DE-FG02-91ER40676 (Boston University), DE-FG03-. 92ER40701 (C a lif ornia Institute of Technology), DE-FG03-93ER40788 (Colorado State University), DE-AC02-76ER03069 (MIT), DE-AC03-76SF00515 (SLAC), DE-FG03-91ER40679 (University of California at Irvine), DE-FG03-94ER40833 (U niversity of Hawaii), DE-FG05-92ER40736 _ (University of Texas at Dallas), DE-AC35-89ER40486 (SSC Lab); by the U.S. National Science Foundation, Grant No. PHY9203212 (University of Washington); and by the Texas National Research Laboratory Commission under Contract Nos. RGFYSlB5, RGFY92B5 (Colorado State), and RCFY93-316H (University of Texas at Dallas). +Present address: Kanas State University, Manhattan, Kanas 66506-2601. tPresent address: Stanford Linear Accelerator Center, Stanford, California 94309. .2 Measurements of the mass, lifetime, and electronic branching fraction of the T lepton have been improved to the extent that they can be used to provide a significant test of lepton universality. The most precise measurement to date of the mass of the 7 lepton is described in this paper. As the third of the sequential charged leptons, the 7 decays similarly to the p by virtue of its coupling to a virtual W boson and a neutrino; however, the r has many more open decay channels as a consequence of its large mass. Within the Standard Model, leptonic decay rates are given by [l] where rnL is the mass of the parent lepton, L, ml is the mass of the daughter lepton, I, GL is the Fermi weak coupling constant, and the correction factor, F,,,, is given by