Study of rare FCNC Lambda_b baryon decays.

罕见的含B夸克强子的FCNC衰变为标准模型之外的新物理提供了高灵敏度的检验。特别是B介子到奇异介子和带相反电荷的轻子对如B_0--> K ~* l+ l ~-的半轻子衰变，近年来受到人们的广泛关注。λ_B重子衰变能提供介子衰变的补充信息。到目前为止，只对λ_B -> λ μ + μ衰变进行了详细的研究.此外，LHC B实验还对衰变Lambda_B --> p K-μ + μ-中的CP破坏进行了研究.在这个项目中，我们的目标是对λ_B --> p K-μ + μ-衰变进行更详细的实验研究.在开始时，微分分支分数将被测量为双μ子不变质量平方的函数。解释对λ_B --> p K-μ + μ-衰变的测量的困难之一来自于这样一个事实，即在这个阶段，没有关于p K-系统的强子结构的理论预测或实验信息。该项目的第二个目标是利用衰变的角分布，目的是首先有助于理解这种衰变的pK结构，并可能测量至少一些对新物理敏感的角观测值。衰变的一个重要特征是它严重依赖于质子和K介子的正粒子识别。在目前的LHCb探测器中，这对于超过10 GeV/c的K介子动量和大约15 GeV/c的质子动量是可能的。这些是相对高的阈值，如果可以实现在较低动量下的颗粒识别，则可以实现灵敏度的显著改善。这种改进不仅有利于本项目的研究，而且LHCb的大量测量也将受益于这种改进，其中一个特别要指出的领域是风味标记。一个建议的解决方案是在LS 3期间安装TORCH探测器，以提供低动量轨道的粒子识别能力。TORCH探测器的设计思想是通过对TORCH石英辐射棒中发射的切伦科夫光子进行精确计时来实现带电粒子的飞行时间测量。初步研究表明，它可以显着提高LHCb探测器的能力。在这个项目中，我们将解决如何将TORCH探测器的信息重建为轨道级粒子识别的高级量的问题。由于TORCH探测器的几何形状，光电探测器看到的切伦科夫图像是复杂的，目前可用的重建需要显着的加速，以供实际使用。我们将利用基于机器学习的替代想法，这可能会加快重建速度，使其在触发级别可用。

The rare FCNC decays of hadrons containing b quark provide highly sensitive tests of new physics beyond the standard model. In particular semileptonic decays of B mesons to strange meson and pair of oppositely charged leptons like B0 --> K* l+ l- received significant attention in recent years. The Lambda_b baryon decays can provide information complementary to the meson decays. Up to now only Lambda_b --> Lambda mu+ mu- decay was studied in the details. In addition, LHCb experiment performed a search for CP violation in the decay Lambda_b --> p K- mu+ mu-. In this project we aim to perform a more detailed experimental study of the Lambda_b --> p K- mu+ mu- decay. At the start, the differential branching fraction will be measured as a function of the dimuon invariant mass squared. One of the difficulties for interpretation of the measurements performed for the Lambda_b --> p K- mu+ mu- decay comes from the fact that at this stage there is no theoretical prediction or experimental information about the hadronic structure of the pK- system. The second goal of the project will be to exploit angular distributions of the decay with the aim to first contribute to the understanding of the pK- structure of this decay and possibly measure at least some of the angular observables which are sensitive to the new physics.The important aspect of the proposed measurements of Lambda_b --> p K- mu+ mu- decay is that it heavily relies on the positive particle identification of protons and kaons. In the current LHCb detector, this is possible for kaon momentum over 10 GeV/c and proton momentum from about 15 GeV/c. These are relatively high thresholds and significant improvement in the sensitivity could be achieved if the particle identification at lower momenta can be achieved. Such improvement would not only benefit study in this project but large number of measurements in LHCb would benefit from such improvement, with one particular area to point out being flavour tagging. One proposed solution is to install a TORCH detector during LS3 to provide particle identification capability for low momentum tracks. The idea of TORCH detector is to exploit time-of-flight measurement for charged particles by precise timing of the Cherenkov photons emitted in TORCH quartz radiator bar. Preliminary studies suggest that it can provide significant improvement in LHCb detector capabilities. In this project, we will attack question of how to reconstruct information from TORCH detector into high-level quantities on particle identification at the track level. Due to the geometry of the TORCH detector, resulting Cherenkov image seen by photodetectors is complex and currently available reconstruction needs significant speedup for practical use. We will exploit alternative ideas based on machine learning, which could possibly speed up reconstruction to the point that it could be usable at the trigger level.