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）项目被规划为下一代能量前沿实验。大型强子对撞机是一种质子-质子对撞机，质心能量为14 TeV，目前正在建设中，将于2007年投入使用。来自日本的100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。大型强子对撞机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失的成分。为了完整地建立模型，必须找到模型并进行详细的研究。通过实验观测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，不仅是为了寻找新的现象，而且对标准模型中包含的参数进行精确测量是大型强子对撞机的另一个重要任务。质子-质子对撞实验的背景必须完全由量子cd来预测。然而，这只是“原则上”。事实上，由于QCD的固有特性，要做出精确的预测是非常困难的。首先，由于QCD具有非常大的耦合常数，摄动级数的收敛性变得非常差，或者有时摄动计算本身就没有意义。而且，即使在摄动适用的区域内，也没有实验确定的好的重整化点。那么，最低级别（树级）的计算对事件率的概率没有任何预测能力。我们需要高阶摄动和非摄动计算来精确预测背景行为。在这个项目中，我们建立了基于微扰QCD构造包含高阶修正的事件生成器的方法。计算的关键成分如下：1)矩阵元I（最低阶），2)矩阵元II（环校正），3)矩阵元III（实辐射），4)环积分，5)软/共线逼近，6)部分-淋浴模型，7)矩阵元与部分淋浴的匹配。根据以上项目的研究，本项目构建了一些简单的事件发生器实例（用于Drell-Yan和prompt光子过程）。现在我们准备计算更复杂的过程，如“WIZ玻色子+射流”。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