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在粒子物理与核物理实验中,探测器模拟起到极其重要的作用。在实验初期,需要通过探测器模拟来优化探测器设计,由实验物理目标确定支撑该目标的探测器配置。在实验运行期间,需要通过探测器模拟作为参考,由从模拟软件提取的探测效率、误差等关键信息将获取的真实数据转换为物理结果。在探测器全模拟中广泛应用的模拟引擎如Geant4[14]等,通过已知的粒子与物质的相互作用数据,用Monte-Carlo方法逐步(step)模拟粒子在探测器中的输运过程。全模拟首先从探测器的详细几何描述入手,加入磁场信息,由Geant4等模拟引擎逐步模拟粒子在各个探测器中的响应,再经过数字化过程,最终输出与真实数据格式一致的模拟数据。该过程对探测器的处理细节繁杂、耗时耗力,并不适用于实验初期探测器种类、几何未定的情况。作为一个快速的方案,参数化的探测器快模拟是现阶段更合适的选择。探测器快模拟,放弃了全模拟中最耗时的输运过程模拟,采用参数化的方法快速模拟粒子在探测器内的响应。探测器的参数作为快模拟的输入,如寻迹探测器的内径、外径、探测层数、单层探测的空间分辨、z向边界范围、探测效率等。模拟过程中,由入射粒子的信息结合所处 位置的探测器信息计算得出该入射粒子的可能探测器观测值。下面我们以径迹探测器为例介绍EicC探测器快模拟软件的开发。
对于径迹探测器,其探测性能可以脱离径迹探测器的类型,由参数化方法进行处理。如径迹探测器的动量分辨可以用式(1)参数化:
$$\begin{split} \left(\frac{\sigma_p}{p}\right)^2 =& \left(\sqrt{\frac{720}{N+4}} \frac{\sigma_{r\phi} p \sin\theta}{0.3BL^2}\right)^2+ \\& \left(\frac{52.3\times10^{-3}}{\beta B\sqrt{LX_0\sin\theta}}\right)^2 + (\cot\theta \sigma_{\theta})^2, \end{split} $$ (1) 其中:等式右侧第一项对应于
$ p_{\rm t} $ 项;第二项对应于多次散射效应;第三项对应于$ \theta $ 项。p为带电粒子动量大小,$ \beta = v/c $ ,B为磁场大小,L为径迹探测区域径向距离,N为径迹探测区域内探测层数,$ X_0 $ 对应于辐射长度,$ \sigma_{r\phi} $ 为探测单元$ r\phi $ 向分辨率,$ \sigma_\theta $ 为探测单元$ \theta $ 向分辨率。图8以径迹探测器的两个参数作为示例展示了动量分辨率对于磁场强度和探测单元层数的依赖关系。EicC快模拟软件在FairROOT框架下[15]进行开发,现有对顶点探测器、径迹探测器、飞行时间探测器、切伦科夫探测器、电磁量能器的参数化快模拟功能。
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摘要: 核子是构成宇宙可见物质的最主要成分,也是研究强相互作用的最佳实验室。对核子内部结构的研究是当前理论和实验研究的重要前沿。在核子内部结构的实验研究中,电子- 离子对撞机(Electron Ion Collider, EIC)是最理想的装置,能提供核子内部最清晰的图像,是人类认识物质世界深层次结构,特别是核子与原子核结构最理想的工具。中国极化电子离子对撞机EicC项目,设想在已开建的HIAF 高能离子束的基础上进行升级:将离子束流升级成15~20 GeV 的极化束流,建设3~5 GeV 高能极化电子束流,实现质心系能量为10~20 GeV双极化电子- 离子对撞,在海夸克能区对核子内部结构进行精细测量,并对质子质量、奇特强子态等诸多重要物理课题展开研究。在本文中,我们开发了EicC 快模拟软件,对探测器性能进行参数化模拟;通过物理模拟汇集EicC探测需求,利用探测器模拟软件进行优化并提出EicC 探测器谱仪的初步设计方案。该谱仪方案提供了接近全立体角的覆盖范围和大动量范围内的粒子鉴别能力,兼顾EicC项目丰富的物理课题。Abstract: Nucleon, the main building block to the visible matter in the universe, is an ideal laboratory to study the strong interaction. In the experimental study of nucleon structure, Electron Ion Collider(EIC) plays important role. EIC is a super electron-microscope being able to take clear image of the inner structure of the nucleon, and hence is an effective tool to gain insights into the fundamental constituents of matter, e.g., especially the structures of the nucleon and nuclei. The Electron-ion collider in China(EicC) project is proposed based on the HIAF facility by promoting its ion beam to an energy around 15~25 GeV, which is then enforced to collide with an electron beam of 3~5 GeV. Both beams are polarized and their center-of-mass energy is 10~20 GeV. The main physics motivations include the precision measurements to the nucleon internal structure in the sea-quark region, and the promotion of our understanding of the origin of proton spin and mass, the study of exotic states, etc. In the paper, a fast simulation package based on parametrisation is developed for EicC. With the simulation package, one EicC detector conceptual design is proposed based on the virous physics simulations.
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Key words:
- EicC /
- QCD /
- nucleon structure /
- exotic state
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