The highly successful Belle experiment was located at the KEKB accelerator in Tsukuba, Japan. KEKB was an electron-positron ring accelerator running at the asymmetric energies of 8 GeV (e-) and 3.5 GeV (e+). The Belle experiment took data from 1999 to 2010, but was shut down in June 2010 in order to begin a major upgrade of the accelerator and the detector. Belle played a crucial role in the award of the 2008 Nobel Prize for Physics to M. Kobayashi and T. Maskawa. The main physics goal of Belle was the measurement of CP-violation in the B-meson system.ududThis mission, as well as the search for physics beyond the Standard Model, has been passed to the Belle II experiment located at the SuperKEKB accelerator, the direct successors of the Belle experiment and KEKB respectively. The precise measurement of CP-violation and the search for rare or "forbidden" decays of the B-meson and the tau-lepton as signals for New Physics relies heavily on a large number of recorded events and the precision with which B-meson and lepton decay vertices can be reconstructed. Thus, the accelerator upgrade aims for an increase of the luminosity by a factor of 40, resulting in a peak luminosity of 8x10^35 cm^{-2} s^{-1}. This upgrade is scheduled to be finished by 2017 and will result in asymmetric beam energies of 7 GeV (e-) and 4 GeV (e+), provided by beams with a vertical size of only 48 nm ("nano-beam optics"), a size that has never been reached at any particle collider before.ududThe accelerator upgrade will result in the desired increase of the collision rate of particles, while it will also inevitably lead to an increase in the background for all sub-detectors. The Belle detector would not have been able to handle the new background conditions expected at SuperKEKB, hence an upgrade of the Belle detector to the Belle II detector was necessary. Additionally the upgrade aims to increase the physics performance of the detector, making it more sensitive to the effects of New Physics. The detector upgrade will see improvements and redesigns of almost all subsystems as well as the inclusion of a whole new sub-detector, the PiXel vertex Detector (PXD). The introduction of the PXD will ensure that decay vertices are reconstructed with an extremely high precision in the harsh background conditions expected at Belle II. The PXD is a semi-conductor based particle tracking detector and will be the innermost sub-detector of Belle II. It offers excellent track and vertex reconstruction capabilities, while having a thickness of only 75 μm in order to minimise multiple scattering effects.ududDue to the innovative concept of a high-luminosity nano-beam accelerator, the scale of background being produced at the future SuperKEKB cannot be derived from a traditional electron-positron collider and has, therefore, to be simulated using first-principle Monte Carlo techniques. This thesis focuses on a detailed study of the expected background for the pixel vertex detector at the upcoming Belle II experiment. It starts with a comprehensive summary of the key components of the SuperKEKB accelerator and the Belle II detector before delving into the details of the Belle II simulation and reconstruction framework basf2. It was decided to develop the basf2 framework from scratch, rather than adapting the software framework used at Belle. The changes made in the upgrade from the Belle to the Belle II detector, would have required major modifications of nearly all existing libraries.ududThis thesis continues by explaining, in detail, the measurement and analysis of an experiment conducted at Belle in 2010, shortly before the KEKB accelerator and the Belle detector were shut down. The experiment aimed at establishing the validity of a major background for the PXD, namely the two-photon process into an electron-positron pair, described by the Monte-Carlo generators KoralW and BDK, which have never been tested in the kinematical region relevant for the PXD. From a comparison based on Monte Carlo data it is found that the difference between KoralW and BDK in the high cross-section, low pt region (smaller than 20 MeV) for the produced electron and positron is very small, and that both Monte-Carlo generators agree with the experiment in this important low momentum regime. However, the question arises as to whether the delivered cross-section of the Monte Carlo generators is correct over a wider phase space, but still below the centre-of-mass energies where these generators have been verified experimentally (e.g. at the e+e- colliders PETRA and LEP). In order to answer this question, a comparison between recorded detector data and Monte Carlo data is performed, an analysis that has never been done for centre-of-mass energies of the order of those of the Belle and Belle II experiments. From the results the conclusion is drawn that both Monte Carlo generators, KoralW and BDK, agree very nicely for low values of pt but differ significantly for intermediate values where the total cross-sections are already very small. The recorded data proved that for intermediate pt ranges the behaviour of BDK is correct, while KoralW overshoots the data. Since, however, the cross-section peaks strongly for low values of pt both generators can be used for further background studies.ududFurthermore, this thesis includes a detailed basf2 simulation study of the major beam and QED backgrounds that are expected at Belle II and their impact on the PXD. Various figures of merit are estimated, such as particle flux, radiation dose and occupancy. On average the inner layer experiences a particle flux of 6.1 MHz cm^{-2} and the outer layer of 2.5 MHz cm^{-2}. The distribution of the particle flux along the global z-axis is fairly flat meaning that the radiation damage is evenly distributed along the PXD ladders. The simulation shows that the inner layer of the PXD is exposed to a radiation dose of 19.9 kGy/smy and the outer layer to a dose of 4.9 kGy/smy. Irradiation tests of DEPFET sensors with 10 MeV electrons showed that the sensors work reliably for a dose of at least 100 kGy. It is believed that they can even cope with up to 200 kGy. Using the radiation dose values obtained from the simulation, the numbers translate to a lifetime of roughly 10 years for the PXD sensors, the typical operation time of a high energy physics detector. The study shows that the expected PXD occupancy, summing over all background sources, is given byududinner layer: 1.28 +- 0.03 %udouter layer: 0.45 +- 0.01 %ududThe upper limit for the PXD, imposed by the data acquisition and the track reconstruction, is 3%. The estimated values are well below this limit and, thus, the PXD will withstand the harsh background conditions that are expected at Belle II.
展开▼
机译:极为成功的Belle实验位于日本筑波的KEKB加速器上。 KEKB是在8 GeV(e-)和3.5 GeV(e +)的不对称能量下运行的电子-正电子环加速剂。 Belle实验收集了1999年至2010年的数据,但为了开始对加速器和检测器进行重大升级,在2010年6月将其关闭。百丽在将2008年诺贝尔物理学奖授予小林先生和正川先生方面发挥了至关重要的作用。 Belle的主要物理目标是在B介子系统中测量CP违规。 ud ud此任务以及对超出标准模型的物理学的搜索已传递给SuperKEKB的Belle II实验加速器,分别是Belle实验和KEKB的直接后继。 CP违反的精确测量以及寻找B介子和tau轻子的稀有或“禁止”衰变作为“新物理学”的信号,在很大程度上取决于大量记录的事件以及B介子和可以重建轻子衰变顶点。因此,加速器升级的目标是将亮度提高40倍,从而使峰值亮度达到8x10 ^ 35 cm ^ {-2} s ^ {-1}。此升级计划于2017年完成,它将产生垂直尺寸仅为48 nm的光束(“纳米光束光学器件”)提供7 GeV(e-)和4 GeV(e +)的不对称光束能量, ud ud加速器升级将导致所需的粒子碰撞率增加,同时也不可避免地导致所有子探测器的背景增大。 Belle检测器将无法处理SuperKEKB预期的新背景条件,因此必须将Belle检测器升级为Belle II检测器。此外,此次升级旨在提高探测器的物理性能,使其对新物理效应更加敏感。检测器升级将对几乎所有子系统进行改进和重新设计,并将包括一个全新的子检测器PiXel顶点检测器(PXD)。 PXD的引入将确保在Belle II预期的恶劣背景条件下以极高的精度重建衰减顶点。 PXD是基于半导体的粒子跟踪探测器,将成为Belle II最里面的子探测器。它具有出色的轨迹和顶点重建功能,同时厚度仅为75μm,以最大程度地减少多重散射效应。 ud ud由于采用了高发光度纳米束加速器的创新概念,因此产生的背景规模为未来的SuperKEKB不能从传统的电子-正电子对撞机中获得,因此必须使用第一原理的蒙特卡洛技术进行模拟。本文重点研究即将进行的Belle II实验中像素顶点检测器的预期背景。在深入探讨Belle II仿真和重建框架basf2的细节之前,它首先全面概述了SuperKEKB加速器和Belle II检测器的关键组件。决定从头开始开发basf2框架,而不是改编Belle使用的软件框架。从Belle升级到Belle II检测器所做的更改将需要对几乎所有现有的库进行重大修改。 ud ud本文将通过详细解释2010年在Belle进行的实验的测量和分析来继续进行,在KEKB加速器和Belle检测器关闭之前不久。该实验旨在确定PXD主要背景的有效性,即由双光子过程转变为电子-正电子对的过程,这是由蒙特卡洛发生器KoralW和BDK所描述的,从未在与之相关的运动学区域进行过测试PXD。从基于蒙特卡洛数据的比较中,发现所产生的电子和正电子在高截面,低pt区域(小于20 MeV)中KoralW和BDK之间的差异非常小,并且蒙特卡洛发电机在这种重要的低动量状态下同意实验。但是,出现了一个问题,即蒙特卡洛发生器的输送横截面在较宽的相空间上是否正确,但仍低于这些发生器经过实验验证的质心能量(例如在e + e处)。 -PETRA和LEP对撞机)。为了回答这个问题,在记录的探测器数据和蒙特卡洛数据之间进行了比较,对于像Belle和Belle II实验那样的质量中心能量,从未进行过分析。从结果可以得出结论,蒙特卡洛发生器,KoralW和BDK,对于pt的低值非常吻合,但对于中间值(其中总横截面已经非常小)却有很大的不同。记录的数据证明,对于中间pt范围,BDK的行为是正确的,而KoralW则超出了数据。但是,由于这两种发生器的pt值较低时横截面峰值强烈,因此可以用于进一步的背景研究。 ud ud此外,本文包括对Belle预期的主要光束和QED背景进行的详细basf2模拟研究II及其对PXD的影响。估计了各种品质因数,例如粒子通量,辐射剂量和占有率。平均而言,内层的颗粒通量为6.1 MHz cm ^ {-2},外层的颗粒通量为2.5 MHz cm ^ {-2}。粒子通量沿全局z轴的分布相当平坦,这意味着辐射损伤沿PXD阶梯均匀分布。模拟显示PXD的内层暴露于19.9 kGy / smy的辐射剂量,外层暴露于4.9 kGy / smy的辐射剂量。用10 MeV电子对DEPFET传感器进行的辐射测试表明,该传感器在至少100 kGy的剂量下能可靠地工作。据信它们甚至可以应对200 kGy。使用从模拟获得的辐射剂量值,这些数字可转换为PXD传感器的大约10年寿命,这是高能物理探测器的典型工作时间。研究表明,在所有背景源上求和的预期PXD占用量由 ud udinner层给出:1.28 +-0.03% udouter层:0.45 +-0.01% ud udPXD的上限由数据采集和轨迹重建为3%。估计值远低于此限制,因此,PXD将承受Belle II预期的恶劣背景条件。
展开▼