Study of Elementary Particles Using OPAL Detector at LEP & LEP II

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The OPAL experiment, in which we have been participating and playing leading roles, is one of the four experiments at the CERN Large Electron-Positron collider (LEP). LEP started its operation in 1989 and was running in the vicinity of the Z^0 boson resonance until 1995. In 1994 and 1995 OPAL collected about 2.7 millions of Z^0 decay events. The following are several main results obtained in recent years using a total of 5 millions of Z^0 events recorded since 1989.1. The Standard Model was tested by measuring various Z^0 properties with unprecedented precisions. The mass of the top quark was estimated, which was later confirmed by its discovery at the Tevatron accelerator.2. A new limit was obtained for the mass of the tau neutrino using a large number of tau particles from the Z^0 decays.3. A variety of bottom quark properties, such as lifetimes of bottomed mesons and baryons and oscillation of neutral bottomed mesons, were measured with various methods and outstandingly good precisions.4. Extensive searches for various new particles were performed.In November 1995 LEP started to increase its collision energy beyond the Z^0 resonance. In 1996 LEP reached the energy which allows pairs of W bosons to be created for the first time in e^+e^- collisions.1. The mass of the W boson has been measured with two independent methods. Even with only a few hundreds of W-pair events observed so far for the four LEP experiments, the accuracy of the mass determination is already comparable to those obtained by other previous experiments.2. The Standard Model is found to be valid at this higher energy region as well.3. The mass of the Higgs boson, an elementary scalar particle predicted in the Standard Model, is found to be heavier than 68.8 GeV.Searches for supersymmetric particles were pursued and the parameters in the theories have been further restricted.