Z0 Line Shape Research Articles

αS from LEP

Recent results on measurements of the strong coupling αS from LEP are reported. These include analyses of the 4-jet rate using the Durham or Cambridge algorithm, of hadronic Z0 decays with hard final state photon radiation, of scaling violations of the fragmentation function, of the longitudinal cross section, of the Z0 lineshape and of hadronic τ lepton decays.

Review of recent neutral current results

The status of neutral current measurements are reviewed with emphasis placed on those measurements related to the tau lepton. These include preliminary results from LEP on the Z0 lineshape using data from the most recent centre-of-mass energy scan in the vicinity of the Z0 resonance; results of the forward-backward asymmetries of the electron, muon and tau lepton and the most recent measurements of the the tau polarisation at LEP. The status of the left-right asymmetry and forward-backward left-right asymmetries for the muon and tau lepton as measured at SLC are summarised. Interpretation of these results in terms of standard model parameters are discussed.

Measurement of the leptonic branching ratios of the τ lepton

The leptonic branching ratios of the tau lepton have been determined from data collected by the OPAL detector in 1991 and 1992. From a sample of 27196 e+e−→τ+τ− candidates we find 7322 $$\tau \to e\nu \bar \nu$$ and 7941 $$\tau \to \mu \nu \bar \nu$$ candidates. Using efficiency and background estimates determined from a study of Monte Carlo events and control samples of data, the branching ratios $$B(\tau \to e\nu \bar \nu )$$ =(18.14±0.20±0.28)% and $$B(\tau \to \mu \nu \bar \nu )$$ =(17.48±0.18±0.23)% have been obtained. These new results have been combined with the published results for the 1990 OPAL data to yield the following branching ratios for data taken between 1990 and 1992: $$\begin{gathered} B(\tau \to e\nu \bar \nu ) = (18.04 \pm 0.33)\% , \hfill \\ B(\tau \to \mu \nu \bar \nu ) = (17.36 \pm 0.27)\% . \hfill \\ \end{gathered} $$ These leptonic branching ratios are used with other properties of the muon and tau-lepton to test the universality of charged current leptonic couplings in these decays. The ratio $$R_\tau = B(\tau \to hadrons + \nu _\tau )/B(\tau \to e\bar \nu _e \nu _\tau )$$ is calculated using our measured values of the leptonic branching fractions of the tau and tau lifetime from which a value of α s (Q 2=M τ 2 ) is extracted. The value of α s (Q 2=M Z 2 ) is obtained byQ 2 evolution and agrees with the value from the Z0 line shape analysis.

The determination of αs (MZ) from the Z0 lineshape and asymmetry data

The derivation of αs (MZ) from lineshape and asymmetry data around the Z0 peak or from the ratio Rl = Γh/Γl, as reported by the LEP Collaborations, is discussed with particular emphasis on the most recent developments in the field of radiative corrections and on a proper treatment of the experimental systematic errors in the error matrix approach. The correlation with the top quark mass is also analysed.