Measurement of the W boson mass with LHCb

该项目旨在利用欧洲核子研究中心大型强子对撞机的LHCb实验首次测量W玻色子的质量。人们普遍认为，在标准模型之外一定存在新的物质物种和力。最有力的发现方法之一是面对SM基本参数之间精确可计算的关系。电弱参数的全局拟合对量子环中重新粒子的可能影响高度敏感。这个测试的灵敏度目前受到测量W玻色子质量的精度的限制。因此，更精确地测量W质量是至关重要的。迄今为止，对W质量最精确的测量来自费米实验室Tevatron质子反质子对撞机的CDF和D0实验。ATLAS和CMS实验的测量正在进行中，ATLAS于2016年12月发布了他们的第一次分析。该测量的不确定度为19 MeV，其中14 MeV归因于W玻色子产生的建模。LHCb在W质量的测量中尚未得到充分利用，但由于其独特的角度覆盖范围，在W质量的最终精度中可能发挥非常重要的作用。与其他探测器覆盖广角粒子产生不同，LHCb聚焦于相对于碰撞质子束的小角度。这意味着许多系统不确定性，特别是与W玻色子产生模型相关的不确定性，将与ATLAS和CMS的不确定性正交，或者在某些情况下与之反相关。LHCb已经拥有了一个庞大的数据集，其中包含数百万个轻子W玻色子衰变，其完整的Run-II数据集将在学生学习期间被记录下来。该数据集将允许测量的统计不确定度约为10兆电子伏。由于其出色的带电粒子跟踪和μ子识别能力，LHCb非常适合于研究μ子W衰变。然而，与ATLAS和CMS相比，LHCb的独特设计意味着这种测量将面临不同的挑战。这将需要新颖的新方法，例如控制背景水平，以及在LHCb覆盖的小角度区域模拟W玻色子的产生。首批目标之一将是精确研究LHCb区域的Z玻色子产生，并将测量到的分布与最先进的理论模型进行比较。这些模型的准确性是测量W质量目标的核心。这是因为从轻子W衰变中得到的带电轻子的运动分布必须与这些工具中的模板进行比较，以便提取W质量。在这方面，Z数据至关重要。这将是一个非常令人兴奋的研究项目，有可能对标准模型的精确零检验的灵敏度产生很大的影响。

This project aims to make the first ever measurement of the W boson mass using the LHCb experiment at the CERN Large Hadron Collider. It is universally accepted that there must be new matter species and forces beyond those of the Standard Model. One of the most powerful discovery approaches is to confront the precisely calculable relations between the fundamental parameters of the SM. A global fit of the electroweak parameters is highly sensitive to possible effects of heavy new particles in quantum loops. The sensitivity of this test is currently limited by the precision with which the W boson mass has been measured. It is therefore of paramount importance that the W mass is measured more precisely. The most precise measurements of the W mass to date come from the CDF and D0 experiments at the Fermilab Tevatron proton antiproton collider. Measurements are underway with the ATLAS and CMS experiments, with ATLAS having released their first analysis in December 2016. This measurement has an uncertainty of 19 MeV, of which 14 MeV is attributed to the modelling of W boson production. LHCb is yet to be exploited in the measurement of the W mass, but could play a very important role in the ultimate precision in the W mass, due to its unique angular coverage. Unlike the other detectors which cover wide angle particle production, LHCb focusses on small angles with respect to the colliding proton beams. This means that many systematic uncertainties, especially those related to W boson production modelling, will be orthogonal to, or in some cases anti-correlated with, those of ATLAS and CMS. LHCb already has a vast dataset with several million leptonic W boson decays, and its full Run-II dataset will have been recorded by midway through the studentship. This dataset will allow a measurement with a statistical uncertainty of around 10 MeV. LHCb is well suited to the study of muonic W decays, thanks to its excellent charged particle tracking, and muon identification capabilities. However, the unique design of LHCb means that different challenges will face this measurement, compared to those of ATLAS and CMS. This will require novel new approaches, for example to control the level of background, and to model W boson production in the small-angle region covered by LHCb. One of the first objectives will be to precisely study Z boson production in the LHCb region, and to compare measured distributions with state-of-the-art theoretical models. The accuracy of these models is central to the goal of measuring the W mass. That is because the kinematic distributions of the charged lepton from leptonic W decays must be compared with templates from these tools, in order to extract the W mass. The Z data will be critical in this regard.This promises to be a very exciting research project, with potential for a high impact on the sensitivity of precision null tests of the Standard Model.