Study of Elementary Particles Using OPAL Detector at LEP-II

欧洲核子研究中心（CERN）世界上能量最高的e+e对撞机LEP在1995年完成了非常成功的Z共振物理项目后，开始了能量升级计划LEP2，以达到W对产生阈值以上的能量。LEP已经逐步更新了最高能量记录。1999年，碰撞能量达到202 GeV，高于最初的预期，并在2000年进一步提高到209 GeV，经过各种努力获得能量。OPAL实验成功地收集了这些新能量下的数据。为了充分利用这些数据，人们在了解探测器的性能、精确校准、发展物理分析以提高搜索的精度和灵敏度以及分析新的物理主题方面做出了努力。在1999-2000年间，OPAL收集了大约8000 W对事件。精确测量了产生截面和W衰变分支比。W玻色子质量的精确测定，加上Z玻色子质量的精确结果，使得标准模式的关键测试成为可能。增加的数据样本可以进一步提高这种测量的准确性。系统地研究了各种标准模型过程，如费米子对的产生和玻色子对的产生。我们发现，标准模型在这些新的能量范围内是有效的。得到了各种新物理模型的约束条件。高能最令人兴奋的事情是可能发现新的粒子。人们一直在寻找希格斯玻色子，但仅从OPAL的数据中还没有找到明确的证据。标准模型希格斯玻色子被发现比110 GeV重。目前正在努力通过结合四个低密度粒子实验的结果来提高灵敏度。已经进行了超对称粒子的搜索。理论中对参数的限制有了很大的改进。OPAL在这两年中发表了大约60篇科学论文。

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.