Neutral current physics with tau leptons using the L3 detector at LEP

In this thesis the analysis of the neutral current process e"+e"-#->##tau#"+#tau#"- has been discussed. Results on measurements of the total cross section, the forward-backward asymmetry, the polarization asymmetry and the forward-backward polarization asymmetry have been presented using the data which were collected with the L3 detector from 1990 to 1992. Special emphasis was given to the measurement of the polarization asymmetry and forward-backward polarization asymmetry. Combining these two measurements A_e and A_#tau# can be extracted. The results are: A_e=0.125#+-#0.022(stat.)#+-#0.013(sys.), A_#tau#=0.171#+-#0.016(stat.)#+-#0.014(sys.). In addition to these results also the measurements of the total cross section and the forward-backward asymmetry in the channels e"+e"-#->#e"+e"-, e"+e"-#->##mu#"+#mu#"-, e"+e"-#->##tau#"+#tau#"- and of the total cross section in the channel e"+e"-#->#q anti q are used to determine the properties of the Z boson. The data are interpreted in the frame work of the Standard Model and of the following model independent ansatzes: the S-matrix, effective couplings and partial width approach. Using exclusively results from the channel e"+e"-#->##tau#"+#tau#"-, the mass and width of the Z boson are measured to be: m_Z=91.189#+-#0.036 GeV, #GAMMA#_z=2.502#+-#0.061 GeV. The effective couplings of #tau# to the Z boson are determined to be: g_V"#tau#=-0.0454#+-#0.0053, g_A"#tau#=0.4993#+-#0.0042. The effective weak mixing angle is derived: sin"2 #THETA#_W = 0.2273#+-#0.0027. The partial decay which of the Z boson into #tau#"+#tau#"- pairs is measured to be: #GAMMA#_#tau#=83.97#+-#0.99 MeV. All measurements are in good agreement with the predictions of the Standard Model. The hypothesis of lepton universality is confirmed. (orig.)