Precise predictions for polarised vector-boson scattering at the LHC

大型强子对撞机(LHC)的头两次运行证实了基本粒子物理标准模型(SM)作为基本相互作用的令人印象深刻的准确描述的有效性。2012年希格斯玻色子的发现和随后对它的许多耦合的测量已经确立了希格斯机制作为对可达能量处的电弱对称破缺(EWSB)的适当描述。尽管如此，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。理解EWSB的起源是基础物理学中最重要的问题之一，也可能与其他大的开放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的机制，有必要仔细研究SM标量部分的动力学。在这方面，矢量玻色子的纵向自由度扮演着特殊的角色，矢量玻色子是通过EWSB从标量扇区产生的。一类对偏离SM特别敏感的过程是纵向矢量玻色子的散射。这些过程只考虑到SM的规范结构和希格斯玻色子的存在。因此，即使是与SM的微小偏差，即反常耦合，也将导致可测量的效应。在大型强子对撞机第二次运行期间，在具有轻子衰变的矢量玻色子的各种通道中测量了未极化的矢量玻色子的散射，并对反常耦合设定了限制。此外，还尝试了半轻子末态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，将允许在矢量玻色子散射中进行偏振测量，在高亮度大型强子对撞机上，纵向矢量玻色子的散射预计将以2-3个标准偏差的精度测量。为了正确地分析和解释这些即将到来的测量，需要足够的理论预测。轻子通道中未极化矢量玻色子散射的电弱修正结果令人惊讶地大，横截面水平为15%，分布水平超过40%。对极化VBS的修正大小预计相似，但可能在细节上有很大不同。这项研究计划的中心目标是计算在大型强子对撞机上极化矢量玻色子散射的精确理论预测。这将用于W玻色子和Z玻色子的散射以及同号W玻色子的散射，包括它们的轻子衰变。此外，我们还针对半轻子通道中的VBS进行了预测，其中W或Z玻色子以强子方式衰变，而W玻色子以轻子方式衰变。

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.