Search for New Physics and the precision determination of the CKM matrix element |Vub| with the Belle II experiment

到目前为止，粒子物理的标准模型成功地描述了自然界中的物质和基本粒子的相互作用。尽管标准模型取得了这一成功，但自然界中观察到的一系列物理现象无法解释，一个突出的例子是宇宙中物质超过反物质。2018年年底，超级风味工厂实验Belle II将开始记录B介子衰变的大样本。B介子的精确研究很有趣，因为它们的衰变可以测量电荷宇称破坏的大小，从而探索粒子衰变中的物质和反物质的不对称性。电荷宇称破坏的大小不能通过一次测量来确定，但由几个精确的观测数据和任何不一致的组合来确定，将是标准模型以外的新物理过程的强烈迹象。这一方案的目的是对CKM矩阵元素VUB进行精确测量，其绝对值是提取电荷宇称破坏大小的重要约束之一。在实验上，可以通过研究半轻子B介子衰变来测量VUB，这些衰变是通过研究排除或包含强子末态而获得的。这两种方法之间长期存在紧张关系，统计意义为3.4标准差。这种张力的一个可能原因是，在使用包含性衰变的最先进测定中，使用模型函数来描述B介子内部的b夸克运动。为了消除这种可能性，本申请提出了一种连贯的策略，如何在用Belle II实验测量的辐射和半轻子B介子衰变的测量运动学分布的全球分析中，与VUB同时提取这种泛函形式。作为筹备工作的一部分，并建立必要的实验技术，将利用Belle数据进行第一次分析。对B介子内部b夸克运动的函数形式的直接实验确定将消除在最先进的包含性VUB测量中所做的模型假设，这反过来将有助于阐明包含性和排他性VUB测量之间的观察到的差异。此外，形函数还可以用来寻找辐射衰变中与模型无关的新的物理贡献。

Matter and interactions of elementary particles in nature are so far successfully described by the Standard Model of particle physics. Albeit this success of the Standard Model, a range of physical phenomena observed in nature cannot be explained and a prominent example is the overabundance of matter over anti-matter in the universe. At the end of the year 2018 the super flavour factory experiment Belle II will start to record large samples of B-meson decays. The precision study of B-mesons is interesting as their decay allows for measuring the size of Charge-Parity violation and thus probe the matter and anti-matter asymmetry in particle decays. The size of Charge-Parity violation cannot be determined by a single measurement, but by the combination of several precision observables and any disagreement would be a strong indication for new physics processes beyond the Standard Model. The goal of this proposal is to carry out a precision measurement of the CKM matrix element Vub, whose absolute value is one of the important constraints in the extraction of the size of Charge-Parity violation. Experimentally Vub can be measured by studying semileptonic B-meson decays obtained by studying exclusive or inclusive hadronic final states. There is a long-standing tension between both approaches with a statistical significance of 3.4 standard deviations. One possible reason for this tension is the use of model functions to describe the b-quark motion inside the B-meson in state-of-the-art determinations using inclusive decays. To eliminate this possibility, this application proposes a coherent strategy how this functional form can be extracted simultaneously with Vub in a global analysis of measured kinematic distributions of radiative and semileptonic B-meson decays measured with the Belle II experiment. As part of the preparatory work and to establish the necessary experimental techniques a first analysis using Belle data will be carried out. The direct experimental determination of the functional form of the b-quark motion inside the B-meson will remove the model assumptions made in the state-of-the-art inclusive Vub measurements which in turn will help to shed light on the observed differences between inclusive and exclusive Vub measurements. In addition the shape function can be used to search for new physics contributions in radiative decays in a model independent way.