Study of Elementary Particles Using OPAL Detector at LEP

我们参与的OPAL实验是欧洲核子研究中心大型正负电子对撞机（LEP）的四个主要实验之一，该实验于1989年开始运行，旨在研究Z^0玻色子共振附近的e^+e^-反应。LEP的表现逐年改善。OPAL实验也非常成功，我们在1992年收集了77万个Z^0事件（仅峰值），在1993年收集了73万个Z^0事件（峰值和非峰值）。从实验开始到现在，Z^0事件的数量已经超过了200万个。随着新探测器的引入以及对探测器的进一步了解，不仅事件的数量增加了，而且数据质量也得到了很大的改善。通过将束流能量的不确定性从20 MeV降低到6 MeV，以最佳精度确定了Z^0质量。通过测量Z^0的性质（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）来检验标准模型。顶夸克的质量被证实约为150 GeV。用测得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检验.通过对γ轻子分支分数和寿命的精确测量，解决了γ轻子的两个问题.对底夸克的各种性质（Z^0部分衰变宽度、对产生的前后不对称性、底介子和重子的寿命以及中性底介子的振荡）进行了高精度的测量.希格斯玻色子，标准模型中的基本标量粒子，被发现比63 GeV重。我们对L3群报道的双光子反常质量峰提出的质量约为60 GeV的新粒子得到了否定的结果。

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.