Achieving electroweak precision at future electron-positron colliders

Abstract

The particle physics community is currently discussing the next large-scale electron-positron collider experiment, with a major interest in the Higgs production threshold around 250 GeV. Multiple proposals have emerged for such a machine, differing in machine and detector designs, as well as energy stages and luminosities. Striving for a machine with the best possible precisions requires both an optimization of each proposal as well a careful examination of the differences between them. The optimization of the individual proposals needs dedicated studies based on detailed simulation of the experiment. Such studies can point out the most critical aspects in the precision, such as e.g. specific steps in the event reconstruction. For understanding the differences between the proposals, a generic approach can be insightful. By neglecting some details and focusing on specific differences, the trade-offs of a choice of one specific collider become clearer. This is especially relevant for the 250 GeV energy stage, which is common to most proposals and central to the physics program. This work contains two studies, corresponding to the two tasks above. The first is a full-simulation study of vector boson scattering (VBS) in the hadronic final state at the 1 TeV stage of the International Linear Collider. This study focuses on the different reconstruction aspects in hadronic final states, which have the highest cross-sections of all W/Z decay modes. A second, more extensive study performs electroweak fits using a generic 250 GeV e+ e− collider with varying beam polarisation and luminosity scenarios. The trade-off between luminosity and availability of beam polarisation is one of the key differences between the proposed colliders. This study investigates how these choices impact the precision on physical and systematic parameters - including the polarisations themselves - and the correlations between physical parameters and systematic effects. It further directly includes a detector systematic to see whether beam polarisation leads to a smaller impact of chirality-independent systematic effects. The body of this thesis gives an overview of the current theory and future collider landscapes, and describes both studies in full detail, including the datasets, methods, and findings. The VBS study finds that the clustering of hadronic final states, the removal of background particles in forward jets, and semi-leptonic decays within jets all significantly degrade the resolution. High level reconstruction remains a limiting factor of electroweak precision, and future studies can look into improving the above named reconstruction aspects to improve the precision in hadronic events. The study of electroweak fits finds that beam polarisation gives access to chiral observables, which allows assumption-free measurement of fermion pair production parameters, and adds significant sensitivity to the measurement of triple gauge couplings. In this context, higher luminosities can only make up for a lack of beam polarisation when introducing a strong set of assumptions. The measurement of beam polarisation in parallel to electroweak parameters with only one polarised beam leads to strong correlations of polarisations with physical parameters, which disappear when both beams are polarised. Finally, the explicitly included detector systematic does not affect any chiral observables, and the fit can extract the exact shape of the systematic effect with high precision. These results show that a careful examination of the impact of beam polarisation is necessary in order to understand the qualitative and quantitative impacts on the achievable precision. Especially the impact on systematic uncertainties will require further studies. For the discussion among different collider proposals, beam polarisation will play a key role and will shape the future of electroweak precision measurements.