Precise predictions for polarised vector-boson scattering at the LHC

大型强子对撞机（LHC）的前两次运行已经证实了基本粒子物理学标准模型（SM）作为对基本相互作用令人印象深刻的精确描述的有效性。2012年希格斯玻色子的发现，以及随后对其许多耦合的测量，已经建立了希格斯机制，作为可及能量下电弱对称性破缺（EWSB）的适当描述。然而，EWSB背后的动力学，包括希格斯玻色子本身的性质，仍然是未知的。了解EWSB的起源是基础物理学中最重要的问题之一，并且可能与其他重大未决问题紧密交织在一起，例如宇宙中物质-反物质不对称的起源以及费米子质量和混合的模式。为了研究EWSB机制，有必要仔细研究SM的标量扇区的动力学。矢量玻色子的纵向自由度在这方面起着特殊的作用，它们是通过EWSB从标量扇区产生的。对偏离SM异常敏感的一类过程是纵向矢量玻色子的散射。这些过程只由于SM的规范结构和希格斯玻色子的存在才尊重统一性。因此，即使是与SM的微小偏差，即异常耦合，也会导致可测量的影响。在LHC的第2次运行中，用轻子衰减的矢量玻色子在不同的通道中测量了非极化矢量玻色子的散射，并确定了异常耦合的极限。此外，还尝试了具有半弛缓最终状态的VBS的首次测量。大型强子对撞机的第三次运行将于2022年开始，将允许对矢量玻色子散射进行偏振测量，在高亮度的大型强子对撞机上，纵向矢量玻色子的散射预计将以2-3个标准差的精度进行测量。为了对这些即将到来的测量进行适当的分析和解释，需要充分的理论预测。在轻子通道中，对非极化矢量玻色子散射的电弱修正结果惊人地大，在横截面上达到15%的水平，在分布上超过40%。对极化VBS的修正预计会有类似的规模，但在细节上可能会有很大的不同。这项研究计划的中心目标是计算出在大型强子对撞机中偏振矢量玻色子散射的精确理论预测。这将用于W和Z玻色子的散射和同符号W玻色子的散射，包括它们的轻子衰变。此外，我们的目标是在半光子通道中预测VBS，其中W或Z玻色子是强子衰变，而W玻色子是轻子衰变。

The first two runs of the Large Hadron Collider (LHC) have confirmed the validity of the Standard Model (SM) of elementary particle physics as an impressively accurate description of fundamental interactions. The discovery of the Higgs boson in 2012 and the subsequent measurement of many of its couplings have established the Higgs mechanism as an appropriate description of electroweak symmetry breaking (EWSB) at the accessible energies. Nonetheless, the dynamics underlying the EWSB, including the nature of the Higgs boson itself, are still unknown. Understanding the origin of EWSB is one of the most important problems in fundamental physics and may be deeply intertwined with other big open questions, such as the origin of the matter-antimatter asymmetry in the universe and the pattern of fermion masses and mixings.In order to investigate the EWSB mechanism, it is necessary to scrutinise the dynamics of the scalar sector of the SM. A particular role in this respect is played by the longitudinal degrees of freedom of the vector bosons, which are generated from the scalar sector via EWSB. A class of processes that is exceptionally sensitive to deviations from the SM is the scattering of longitudinal vector bosons. These processes respect unitarity only owing to the gauge structure of the SM and the presence of the Higgs boson. As a consequence, even small deviations from the SM, i.e. anomalous couplings, will lead to measurable effects.During run 2 of the LHC the scattering of unpolarised vector bosons has been measured in various channels with leptonically decaying vector bosons, and limits on anomalous couplings have been set. In addition first measurements of VBS with semileptonic final states have been attempted. Run 3 of the LHC, which shall start in 2022, will allow for polarisation measurements in vector-boson scattering, and at the high-luminosity LHC the scattering of longitudinal vector bosons is expected to be measured with an accuracy of 2-3 standard deviations. For a proper analysis and interpretation of these upcoming measurements adequate theoretical predictions are required.The electroweak corrections to the scattering of unpolarised vector bosons in the leptonic channels turned out to be surprisingly large, at the level of 15% for cross sections and more than 40% in distributions. The corrections to polarised VBS are expected to be of similar size, but could differ substantially in detail.The central goal of this research proposal is the computation of precise theoretical predictions for the scattering of polarised vector bosons at the LHC. This shall be done for the scattering of W and Z bosons and the scattering of like-sign W bosons including their leptonic decays. In addition we aim at predictions for VBS in the semileptonic channel, where a W or Z boson decays hadronically and a W boson leptonically.