Study of rare FCNC Lambda_b baryon decays.

含有b夸克的强子的罕见FCNC衰变为超越标准模型的新物理提供了高灵敏度的测试。特别是B介子到奇异介子的半光子衰变，以及B0—> K* l+ l-等对带相反电荷的轻子，近年来受到了极大的关注。Lambda_b重子衰变可以提供与介子衰变互补的信息。目前对λ da_b—> λ mu+ mu-衰变进行了详细的研究。此外，LHCb实验还在λ da_b—> p K- mu+ mu-衰变中进行了CP违逆的搜索。在这个项目中，我们的目标是对Lambda_b -> p K- mu+ mu-衰变进行更详细的实验研究。开始时，微分分支分数将被测量为二维不变质量平方的函数。解释Lambda_b—> pK- mu+ mu-衰变的测量结果的困难之一来自这样一个事实，即在这个阶段没有关于pK-系统强子结构的理论预测或实验信息。该项目的第二个目标将是利用衰减的角度分布，目的是首先有助于理解这种衰减的pK-结构，并可能测量至少一些对新物理敏感的角度可观测值。提出的Lambda_b—> p K- mu+ mu-衰变测量的重要方面是，它严重依赖于质子和介子的正粒子识别。在目前的LHCb探测器中，这是可能的，因为介子动量超过10 GeV/c，质子动量约为15 GeV/c。这是相对较高的阈值，如果能够在较低的动量下实现粒子识别，则可以显著提高灵敏度。这种改进不仅有利于本项目的研究，而且LHCb中的大量测量也将受益于这种改进，其中一个特别需要指出的领域是风味标签。一种建议的解决方案是在LS3期间安装TORCH探测器，以提供低动量轨迹的粒子识别能力。TORCH探测器的想法是利用飞行时间测量带电粒子，通过精确定时的切伦科夫光子发射在TORCH石英辐射棒。初步研究表明，它可以显著提高LHCb探测器的能力。在这个项目中，我们将研究如何将TORCH检测器的信息重构为轨道级粒子识别的高水平量。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加快实际使用。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，使其在触发层面上可用。

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.