Study of Elementary Particles Using OPAL Detector at LEP-II

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The world highest energy e+e- collider LEP at CERN has started the energy upgrade program LEP2 after the completion in 1995 of its highly successful physics program at the Z resonance, to the energy above the threshold of W pair production. LEP has progressively updated the highest energy record. In 1999 the collision energy reached at 202 GeV, higher than originally anticipated, and was further pushed up in 2000 to 209 GeV after every efforts to gain energy had been made. The OPAL experiment successfully collected data at these new energies. In order to make full use of these data, efforts have been made for understanding the detector performance, precise calibration, development of physics analysis to gain precision and sensitivity to searches, and analyses of new physics topics have been purseed.In 1999-2000, OPAL collected about 8000 W pair events. The production cross-section and W decay branching ratios have been accurately measured. Precise determination of the W boson mass, together with the precision results of Z boson, allows crucial tests of the Standard Mode. The increased data sample allows the accuracy of such measurements to be further improved.Systematic studies have been made for variety of Standard Model processes, such as fermion-pair production and boson-pair production. The Standard Model is found to work well at these new energy region. Constraints on various models of new physics have been obtained.The most exciting thing with higher energy is a possible discovery of new particles. Higgs boson has been searched for, but no clear evidence has been found in the OPAL data alone. The Standard Model Higgs boson is found to be heavier than about 110 GeV.Effort to gain sensitivity by combining results of four LEP experiments is being pursued. Searches for Supersymmetric particles have been performed. Limits on the parameters in the theory have been largely improved.OPAL has published about 60 scientific papers in these two years.