Study of charm mixing and CP violation with the LHCb experiment

本文研究的主要目的是测量中性粲介子（'D'）系统的混合和寻找间接CP违背。这一目标将通过多体衰减的时间相关的达利兹图研究来实现，最初的重点是通道D->K^0_S KK。分析需要了解达利茨空间上强相的变化，这些信息将从在魅力阈值（即CLEO-c和BESIII）的实验中进行的测量中获得。这种方法将使研究避免任何模型依赖，并将允许结果以一种干净和直接的方式解释。测量的目的具有很高的科学重要性。魅力混合是用两个参数来描述的，x_D和y_D，这两个参数仍然鲜为人知。实际上，尚不确定x_D是否为偶非零。据预测，粲粲子的间接CP违背在标准模型中非常小，任何以当前实验精度进行的观察都有强有力的理由被解释为一种新物理学驱动的效应。LHCb积累的粲衰变样本比以往任何设施都要大得多，因此提供的分析机会是前所未有的，非常令人兴奋。以前对D0->K^ 0_skk进行的混合研究很少，对LHCb的混合研究也没有。衰变的共振结构与D0->K^0_S pipi的共振结构非常不同，这本身就是LHCb内部正在进行的分析的主题，使两个通道在物理范围上互补。（事实上，学生将与研究K^0_S pipi的团队密切合作，这将允许在两种分析之间交换思想和方法）。以往对D0->K^ 0_skk的研究均未采用与模型无关的方法，本文将采用这一方法进行分析。存在将测量程序扩展到其他D衰减通道的可能性，再次追求与模型无关的方法。一个很好的候选者是模式D->Kpipipi，它具有丰富的中间共振结构，并且在LHCb和CLEO-c的先前研究中，东道国研究所存在专业知识。如前所述，该学生将与其他物理学家（来自牛津大学和欧洲核子研究中心）一起工作，他们已经开始对姊妹衰变进行类似的分析。他还将受益于牛津大学最近与北京BESIII实验的合作。在BESIII上进行的“工程测量”对于利用LHCb数据集的全部统计能力将是无价的。最后，学生有可能开发出校准、重建和分析工具，适用于用LHCb Upgrade分析第一批数据。该实验将于2021年开始数据采集，学生可能可以使用升级的早期数据进行首批物理测量之一。在学生学习的第二年晚些时候，将决定是否继续走这条路，或者是否使用现有的数据集进行更多的研究。这项工作将算作与主要论文目标一起的次要项目，如上所述。

The primary purpose of the thesis research is to perform a measurement of mixing and a search for indirect CP violation in the neutral charm meson ('D') system. This goal will be achieved through a time-dependent Dalitz plot study of multibody decays, initially focusing on the channel D->K^0_S KK. The analysis requires knowledge of the variation of the strong phase over Dalitz space, and this information will be taken from measurements performed by experiments at charm threshold (i.e. CLEO-c and BESIII). This approach will enable the study to avoid any model-dependence, and will allow the results to be interpreted in a clean and straightforward manner.The aims of the measurement are of very high scientific importance. Charm mixing is described by two parameters, x_D and y_D, that are both still poorly known. Indeed it is not yet established whether x_D is even non zero. Indirect CP violation in charm is predicted to be very small in the Standard Model, and any observation made with current experimental precision would have a strong claim to be interpreted as a New Physics driven effect. LHCb has accumulated samples of charm decays much larger than at any previous facility, and hence the opportunities presented for analysis are unprecedented and very exciting.There have been very few previous mixing studies performed with D0->K^0_S KK, and none on LHCb. The resonance structure of the decay is very different to that of D0->K^0_S pipi, which is itself the subject of an ongoing analysis within LHCb, making the two channels complementary in their physics reach. (Indeed the student will collaborate closely with the team working on K^0_S pipi, which will allow ideas and methods to be exchanged between the two analyses). No previous study of D0->K^0_S KK has adopted the model independent approach that will be pursued in the thesis analysis.The possibility exists to extend the measurement programme to other D decay channels, again pursuing the model-independent approach. One good candidate is the mode D->Kpipipi, which has a rich structure of intermediate resonances, and where expertise exists in the host institute from previous studies performed at LHCb and CLEO-c.As stated, the student will work together with a team of other physicists (from Oxford and CERN), who are already embarked on a similar analysis with a sister decay. He will also benefit from the association that Oxford has recently made with the BESIII experiment in Beijing. 'Engineering measurements' performed at BESIII will be invaluable in harnassing the full statistical power of the LHCb data set.Finally, it is possible that the student will develop calibration, reconstruction and analysis tools suitable for analysing first data with the LHCb Upgrade. This experiment will begin data taking in 2021, and it may be feasible for the student to perform one of the first physics measurements with the early data from the Upgrade. A decision will be made late in the second year of the studentship whether this path will be pursued, of whether more studies will be made with the existing data set. This work would count as a secondary project alongside the major thesis goal, described above.