Development of the event generator for LHC including higher order corrections

欧洲核子研究中心（CERN）的大型强子对撞机（LHC）项目计划作为下一代能源前沿实验。LHC加速器是一台质心能量为14 TeV的质子-质子对撞机，正在建设中，将于2007年投入使用。来自日本的一百多名物理学家参加了其中一个实验小组（ATLAS合作），并正在为即将到来的实验做准备。LHC实验的物理动机之一是发现希格斯粒子，这是标准模型中唯一缺少的成分。为了建立完整的模型，有必要找到它并进行详细的研究。通过实验观察与理论预测的微小偏差来寻找标准模型之外的新现象也应该是重要的。此外，不仅寻找新现象，而且对标准模型中包含的参数进行精确测量也是另一个重要的ISS ...更多信息 质子-质子碰撞实验的本底必须用QCD完全预言。但这只是“原则上”。事实上，由于QCD的一个固有特性，要做出精确的预言是非常困难的。首先，由于QCD具有很大的耦合常数，微扰级数的收敛性变得很差，或者有时微扰计算本身是没有意义的。此外，即使在微扰适用的区域，也没有实验上定义好的重整化点。那么最低层（树层）计算对事件率的概率没有任何预测能力。我们需要高阶微扰和非微扰计算来精确预测背景行为，在本项目中，我们建立了基于微扰QCD构造包含高阶修正的事件发生器的方法。计算的关键要素如下：1）矩阵元素I（最低阶），2）矩阵元素II（循环校正），3）矩阵元素III（真实的辐射）、4）环积分、5）软/共线近似、6）部分子簇射模型、7）矩阵元与部分子簇射之间的匹配。在这个项目中，已经构造了一些简单的事件发生器（用于Drell-Yan和瞬发光子过程）的例子。现在我们已经准备好计算更复杂的过程，比如“WIZ玻色子+喷流”。少

The LHC(Large Hadron Collider) project at CERN is planned as the next-generation energy-frontier experiment. The LHC accelerator, which is a proton-proton colliding machine with the center-of-mass energy 14 TeV is under construction toward a beam commissioning in 2007. From Japan, more than hundred physicists participate to one of experimental groups (ATLAS collaboration), and are preparing for the coming experiments.One of physics motivations of the LHC experiments is to discover the Higgs particle, which is the only one missing ingredient in the standard model. In order to establish the model completely it is essential to find it and to investigate in details. It should be also important to search for new phenomena beyond the standard model through a tiny deviation of the experimental observations from theoretical predictions. Further not only for searching for new phenomena, but also performing precise measurement of parameters included in the standard model is another important iss … More ue of LHC.The background of the proton-proton colliding experiment must be completely predicted by QCD. This is, however, only 'in principle'. In fact it is very difficult task to make precise predictions because of an inherent feature of QCD. First of all, as QCD has a very large coupling constant, the convergence of perturbation series becomes very poor, or sometimes a perturbative calculation itself is meaningless. Moreover even in the region where the perturbation is applicable, there is no good renormalization point well defined experimentally. Then the lowest level(tree-level) calculation does not have any predictive power for the probability of the event rate. We need both higher-order perturbation and non-perturbative calculations for precise predictions of the background behavior.In this project we have established the method how to construct the event generator that includes the higher-order corrections based on the perturbative QCD. The key ingredients of calculation are as follows;1) Matrix Element I (lowest order),2) Matrix Element II (loop correction),3) Matrix Element III (real radiation),4) Loop Integration,5) Soft/Collinear Approximation,6) Parton-Shower model, 7) Matching between the matrix element and the parton shower.According to the study of above items, some simple examples of event generators (for Drell-Yan and prompt photon processes) have been constructed in this project. Now we are ready to calculate more complex processes like 'WIZ boson + jets'. Less