Probing the flavour structure of New Physics with Bs mesons at LHCb

到目前为止，宇宙最令人困惑和无法解释的特征之一是没有反物质。在宇宙大爆炸期间，物质和反物质以等量产生。然而，通过对宇宙射线的测量，我们知道我们的星系主要由物质组成。此外，没有证据表明，在有反物质云的遥远星系中，物质和反物质湮灭后会产生强烈的伽马射线发射。在早期的宇宙中，一定存在着区分物质和反物质的过程。俄罗斯物理学家萨哈罗夫证明了必须满足三个条件。其中之一是粒子和反粒子在电荷共轭（粒子和反粒子的交换）和宇称（左右镜像对称）称为CP的联合作用下应该表现不同。在粒子物理的标准模型中预测会发生CP违反。到目前为止，所有的测量结果都与标准模型相符。然而，标准模型中CP违背的程度太小，无法解释我们宇宙中物质的主导地位。一种可能性是，新物理学（如超对称）导致了CP违反的额外来源。为了解决这个难题，需要更精确的测量。如果发现与标准模型相比的差异，这将是新物理学的有力证据，也将限制它的形式。在欧洲核子研究中心开始运行的大型强子对撞机将开启物理学的新时代。大型强子对撞机美容实验（LHCb）是在大型强子对撞机上进行的一项特殊目的实验。这项合作由来自全球52个研究所的约400名物理学家组成。通过非常精确地研究含有b夸克和反b夸克的衰变速率的差异，它将提供对CP不对称现象的深入了解。Bs混合中的CP违背是标准模型的一个基本预测。混合相是相对不受约束的。最近，CDF和D0对该参数进行了首次测量。尽管不确定性很大，但它们倾向于比标准模型预测的值大得多的值。LHCb将对这一数量进行精确测量，要么揭示新物理学的存在，要么证实标准模型的预测。独特的是，该实验能够在许多衰减模式下进行测量，从而允许对数据的一致性进行重要的交叉检查并进行分析。本文的主要目的是在LHCb中测量Bs混合相。我将首先测量这个量使用黄金模式b ->J/psi。该模式统计量大，且相对容易触发和重构。这将是该实验的重要早期测量。接下来，我将在模式b -> J/Psi下测量这个量，同时研究通道Bd ->。这些模式很难重建，产量也较低，需要数年时间才能收集到具有统计意义的样本。提高产量的一种方法是在更高的亮度下进行实验。LHCb计划在2015年左右升级实验的触发器和电子设备，以便在更高的亮度下运行并收集更多的统计数据。较高的亮度将导致触发和重建环境更加繁忙，同时也会增加探测器遭受的辐射损伤。我将致力于以最小化探测器中材料量为目标的全局探测器优化研究。

One of the most puzzling and so far unexplained features of the Universe is the absence of anti-matter. During the Big Bang matter and anti-matter were generated in equal quantities. However, from measurements of cosmic rays it is known that our galaxy consists mainly of matter. Furthermore, there is no evidence for the intense gamma ray emission that would follow the annihilation of matter and anti-matter in distance galaxies with clouds of antimatter. In the early Universe processes must have existed that distinguished between matter and anti-matter. The Russian physicist Sakharov showed that three conditions have to be fulfilled. One of these is particles and anti-particle should behave differently under the combined operation of charge conjugation (swapping of particle and anti-particle) and parity (left-right mirror symmetry) called CP. In the Standard Model of particle physics CP violation is predicted to occur. All measurements made so far agree with the Standard Model picture. However, the level of CP violation in the Standard Model is much too small to explain the observed dominance of matter in our Universe. One possibility is that New Physics such as Supersymmetry leads to additional sources of CP violation. In order to resolve this puzzle more precise measurements are needed. If discrepancies, compared to the Standard Model picture, are found this will be strong evidence for New Physics and also constrain its form. The Large Hadron Collider, which is starting operation at CERN, will open a new era in physics. The Large Hadron Collider Beauty Experiment (LHCb) is a special purpose experiment running at the LHC. The collaboration consists of around 400 physicists from 52 institutes world-wide. By studying very precisely differences in the rate of decays containing b and anti-b quarks it will provide insight into the phenomenon of CP asymmetries. CP violation in Bs mixing is a fundamental prediction of the Standard Model. The mixing phase, beta_s, is relatively unconstrained. Recently, first measurements of this parameter have been made by CDF and D0. Though the uncertainties are large they favour values much larger than predicted by the Standard Model. LHCb will make precise measurements of this quantity and will either reveal the presence of New Physics or confirm the Standard Model prediction. Uniquely, the experiment is able to make measurements in many decay modes allowing important cross-checks of the consistency of the data and analysis to be made. The main aim of this proposal is the measurement of the Bs mixing phase at LHCb. I will first measure this quantity using the golden mode Bs->J/psi phi. The statistics in this mode are large and it is relatively easy to trigger and reconstruct. It will be an important early measurement for the experiment. Following this, I will measure this quantity in the mode Bs-> J/Psi eta' and also study the channel Bd -> phi phi. These modes are harder to reconstruct and have lower yields and it will take several years to collect a statistically significant sample. One way to increase the yields will be to run the experiment at higher luminosity. LHCb is planning to upgrade the trigger and electronics of the experiment around 2015 in order to run at higher luminosities and to collect even higher statistics. The higher luminosity will result in a busier environment for triggering and reconstruction as well as increasing the radiation damage suffered by the detector. I will contribute to studies of the global detector optimization with the aim of minimizing the amount of material in the detector.