Results of Electroweek Interaction from Lep-ii Experiment at Cern

Experiments with the Large Electron-Positron Collider at CERN have been started in 1989. Four large detectors have been constructed at LEP: ALEPH, DELPHI, L3, and OPAL. The simultaneous operation of all these detectors provided powerful cross checks of the obtained results that made them highly reliable. PNPI took an active part in construction and operation of the L3 detector. In the first period of 1989 – 1995, LEP operated at the center-of mass energy of mz (LEP-I phase). The main motivation was investigations of electroweak (EW) processes and verification of the validity of the most complete theory – the Standard Model (SM). The first target was the intermediate neutral vector boson, already experimentally discovered, Z boson. In the six years period several millions of Z’s was produced, its mass and widths (total and partial), branching ratios into all possible final states were measured. The SM parameters were determined with high accuracy. There was no evidence of deviations from the SM predictions. Some free parameters of the theory were estimated. The number of leptons families was established to be equal to three, the same as the number of quarks families in the strong interaction sector of the SM. The high precision reached provided a possibility to constrain the mass of not yet discovered top quark from the calculations of the loop correction to the bare Z-boson mass using in the calculations the value of the charged vector boson mass mw, already observed in the experiments on hadron colliders. The correction is rather strongly, quadratically, depends on the quark mass, but there are also contributions from the scalar bosons. To explain the mass splitting of the EW intermediate bosons – photon, Z and W − there should be at least one scalar Higgs boson (H), which also enters into the loop calculations. It was not found any scalar boson in the experiments, i.e. the mass of the boson is high (mH > 58 GeV), moreover the loops correction from this source is only weakly (logarithmically) dependent on the Higgs boson mass. For this reason, the prediction of the mass of top quark was rather accurate ( 30 16 197 t m + − = GeV). Such low limit gave a chance to observe the top quark production on the proton-antiproton collider at Fermilab. After several years of very difficult studies the top quark was discovered by Fermilab experiments, confirming once more the high predictive power of the SM. The main results of the L3 experiments during LEP-I operation have been already presented in the previous reports [1, 2]. Already at the early stage of the LEP project, it was decided to rise up the beam energy to 100 GeV (LEP-II). At this energy the EW cross section of production of the W pairs is rising first and then is flattened out. Copious W production studies make possible to determine the parameters of the W boson with higher accuracy, making the theoretical constraints much tougher, in particular predicting the range of the Higgs boson mass. The data taking on LEP-II was terminated in 2000 but sophisticated analysis is still under way. For example, during last 5 years the L3 collaboration has published more than 30 papers with the final results. At the same time, all four LEP experiments – ALEPH, DELPHI, L3, and OPAL – in the frame of the LEP Working groups (WG) – carried out combined studies of different topics of the SM, with consideration of all available data including the relevant Fermilab results. Some of the WG reports are already published or presented during international conferences. In this report, the most spectacular results from the LEP-II experiments on the SM verifications will be reviewed [3].