Master Thesis defense by Johanna Helena Jallberg

Title: Luminosity Measurements at the FCC-ee

Abstract: High energy collision experiments have historically contributed immensely to our understanding of the fundamental forces and particles shaping our universe, by testing, confirming and pushing the boundaries of the Standard Model. The proposed Future Circular Collider (FCC) represents the next milestone in this journey, beginning with a high-luminosity e+e¯collider known as the FCC-ee, which already in its first phase offers the potential for unparalleled measurements of the Z-lineshape. For this purpose, achieving a systematic precision of 10¯⁴ on the absolute luminosity is essential. This is facilitated by a pertaining carefully designed luminosity calorimeter (LumiCal), likely using low angle Bhabha scattering as reference process.

This thesis is an in depth study of this detector, exploring both its capabilities and challenges. A full simulation of LumiCal forms the basis for detailed studies of detector response to various event types, as well as development and discussion of methods for event reconstruction. The analyses provides an assessment of key baseline characteristics of the detector, such as a sampling fraction of 1.1%, a 3.2% accuracy on energy measurements, and a spacial resolution of 75.4 µm. The impact of systematic effects are also considered, and the findings include a systematic magnetic field induced trajectory deflection, and a significant smearing of the polar scattering angle due to multiple scattering in the beam pipe, ultimately responsible for a 111.7 µm contribution to the spatial resolution.

Furthermore, events from the reference process and background from beam-beam interactions source is characterised, and the associated detector response studied. In particular, the background is estimated to contribute a problematic energy deposition in the detector, corresponding to 2% of that of the reference process.