MASHIMO Tetsuo The University of Tokyo, Center for Elementary Particle Physics, Assistant, 素粒子物理国際研究センター, 助手 (60181640)
MORI Toshinori The University of Tokyo, International Center for Elementary Particle Physics, A, 素粒子物理国際研究センター, 助教授 (90220011)
KAWAMOTO Tatsuo The University of Tokyo, International Center for Elementary Particle Physics, A, 素粒子物理国際研究センター, 助教授 (80153021)
KOMAMIYA Sachio The University of Tokyo, International Center for Elementary Particle Physics, P, 素粒子物理国際研究センター, 教授 (80126060)
ORITO Shuji The University of Tokyo, Graduate School of Science, Professor, 大学院・理学系研究科, 教授 (10092173)
The world highest energy e^+e^- collider LEP at CERN has started its operation in 1989. After completion in 1995 of highly successful physics program at the Z resonance, LEP has started the energy upgrade program LEP2 in. 1996, exceeding the collision energy sufficient to produce the W boson in pair. In 1997 the LEP collision energy was increased to 183 GeV and further pushed up to 189 0eV in 1998. The OPAL experiment, in which we have been playing a leading role, has successfully collected data at this new energy region. In order to make full use of these data, we worked on understanding the detector performance at high energy, precise calibration of the detector, developments for improved physics analysis technique to gain precision of measurements and sensitivity for searches, and we also pursued a number of new physics topics.
1. About 4000 W pairs events have been collected. The production cross-section, total W width and decay branching ratios were precisely measured. Accurate measurement of the W mass, together with the precision results of the Z boson, provides information for crucial tests of the Standard Model. Based on the increased data sample of W pair events, precision of W mass determination has been largely improved.
2. Systematic investigations were made for varieties of Standard Model processes such as fermion pair production, Z pair production, single W production and general four-fermion final states. The Standard Model is found to work well at this higher energy region.
3. The mass of the Higgs boson, which is a key element of the Standard Model yet undetected, is found to be heavier than about 91 GeV.
4. Searches for Supersymmetric particles were performed and improved limits on the parameters in the theory have been obtained.