Study of Elementary Particles Using OPAL Detector at LEP

我们正在参与的欧普实验是欧洲核子研究中心大型电子-正电子对撞机(LEP)的四个主要实验之一，LEP于1989年开始运行，研究Z^0玻色子共振附近的e^+e^-反应。LEP的业绩一直在逐年提高。Opal实验也非常成功，我们在1992年收集了770k Z^0事件(仅峰值)，在1993年收集了730k Z^0事件(高峰和非高峰)。从实验开始到现在，Z^0事件的数量已经超过了200万次。不仅增加了事件的数量，而且通过引入新的探测器元件以及对探测器的更好的理解，数据质量也得到了很大的改善。通过将束流能量的模糊度从20 MeV减小到6 MeV，以最佳精度确定了Z^0质量。用Z^0性质(总衰变宽度、部分衰变宽度、轻子对和夸克对产生的前后不对称性)的测量对标准模型进行了检验。顶夸克质量被确认为约150GeV。用测量的电弱相互作用和强相互作用的耦合常数对大统一理论进行了验证。通过精确测量伽马轻子的分支分数和寿命，解决了关于伽马轻子的两个问题。测量了底夸克的各种性质(Z^0部分衰变宽度、产生对的前后不对称性、底介子和重子的寿命以及中性底介子的振荡)，测量精度非常高。希格斯玻色子，标准模型中的基本标量粒子，被发现比63GeV重。对于质量约为60GeV的新粒子，我们得到了否定的结果，这是由L3群报道的两个光子的反常质量峰所表明的。

The OPAL experiment, in which we are participating, is one of the four major experiments at the CERN large electron-positron collider(LEP), which started operation in 1989 to study e^+e^- reactions in the vicinity of the Z^0 boson resonance. The performance of LEP has been improving year after year. The OPAL experiment has been also very successful, and we collected 770k Z^0 events(peak only)in 1992 and 730k Z^0 events(peak and off-peak)in 1993, respectively. The number of Z^0 events from the beginning of the experiment now amounts to more than 2 million events. Not only the number of events is increased, but also the data quality is much improved with newly introduced detector elements as well as with better understanding of the detector.The important results are listed below.1. The Z^0 mass was determined with the best precision, by reducing the ambiguity of the beam energy from 20 MeV to 6 MeV. The standard model was tested with the measurements of Z^0 properties (total decay width, partial decay widths, forward-backward asymmetries of lepton-pair and quark pair productions). The top quark mass was confirmed to be about 150 GeV. The grand unified theory was tested with the measured coupling constants of the electroweak and strong interactions.2. Two problems on gamma lepton were solved with the precise measurements of the branching fractions and the lifetime of gamma lepton.3. A variety of bottom quark properties (Z^0 partial decay width, forward-backward asymmetry of pair production, lifetimes of bottomed mesons and baryons, and oscillation of neutral bottomed meson)were measured with outstandingly good precision.4. The Higgs boson, elementary scalar particle in the standard model, is found to be heavier than 63 GeV. We obtained negative result on new particle with about 60 GeV mass, which had been suggested by the anomalous mass peak of two photons reported by the L3 group.