Study of Elementary Particles Using OPAL Detector at LEP & LEP II

欧帕尔实验是欧洲核子研究中心大型电子-正电子对撞机(LEP)的四个实验之一，我们一直参与并发挥着主导作用。LEP于1989年开始运行，直到1995年一直在Z^0玻色子共振附近运行。在1994和1995年，Opal收集了大约270万次Z^0衰变事件。以下是近年来利用1989.1以来总共记录的500万次Z^0事件所获得的几个主要结果。标准模型通过测量各种Z^0属性以前所未有的精度进行了测试。顶夸克的质量得到了估计，这一点后来在泰瓦特龙加速器上的发现中得到了证实。利用来自Z^0衰变的大量的tau粒子，得到了tau中微子质量的一个新的极限。用不同的方法测量了底夸克的各种性质，如底介子和重子的寿命以及中性底介子的振荡，都有很好的精度。对各种新粒子进行了广泛的搜索。1995年11月，LEP开始增加其碰撞能量，使其超出Z^0共振。1996年，LEP达到了在e^+e^-碰撞中第一次产生W玻色子对的能量。用两种独立的方法测量了W玻色子的质量。即使到目前为止在四个LEP实验中只观测到几百个W对事件，质量测定的精度也已经可以与其他先前实验获得的结果相媲美。标准模型在这个高能区也是有效的。希格斯玻色子是标准模型中预言的一种基本标量粒子，其质量比68.8GeV重。人们继续寻找超对称粒子，并进一步限制了理论中的参数。

The OPAL experiment, in which we have been participating and playing leading roles, is one of the four experiments at the CERN Large Electron-Positron collider (LEP). LEP started its operation in 1989 and was running in the vicinity of the Z^0 boson resonance until 1995. In 1994 and 1995 OPAL collected about 2.7 millions of Z^0 decay events. The following are several main results obtained in recent years using a total of 5 millions of Z^0 events recorded since 1989.1. The Standard Model was tested by measuring various Z^0 properties with unprecedented precisions. The mass of the top quark was estimated, which was later confirmed by its discovery at the Tevatron accelerator.2. A new limit was obtained for the mass of the tau neutrino using a large number of tau particles from the Z^0 decays.3. A variety of bottom quark properties, such as lifetimes of bottomed mesons and baryons and oscillation of neutral bottomed mesons, were measured with various methods and outstandingly good precisions.4. Extensive searches for various new particles were performed.In November 1995 LEP started to increase its collision energy beyond the Z^0 resonance. In 1996 LEP reached the energy which allows pairs of W bosons to be created for the first time in e^+e^- collisions.1. The mass of the W boson has been measured with two independent methods. Even with only a few hundreds of W-pair events observed so far for the four LEP experiments, the accuracy of the mass determination is already comparable to those obtained by other previous experiments.2. The Standard Model is found to be valid at this higher energy region as well.3. The mass of the Higgs boson, an elementary scalar particle predicted in the Standard Model, is found to be heavier than 68.8 GeV.Searches for supersymmetric particles were pursued and the parameters in the theories have been further restricted.