ATLAS

Abstract:

The question of the nature of the universe at its most fundamental level is one which has been with humankind since ancient times. The development of the theoretical basis of the Standard Model of elementary particles during the 20th century, and its subsequent experimental validation, is a triumph of modern physics. However, the Standard Model is known to be incomplete, and, in the Large Hadron Collider era at the high-energy and high-intensity frontier, our understanding and modelling of the Standard Model has increasingly become limiting for searches for physics beyond it. In this thesis, a measurement performed using 36.2 fb−1 of integrated luminosity recorded by the ATLAS experiment during Large Hadron Collider Run 2 of a crucial benchmark process for Standard Model modelling is presented; W boson production in association with jets. This is the first such measurement in the W → μν decay channel at √s =13 TeV. Differential cross-sections, in observables sensitive to the modelling of quantum chromodynamics, are presented for both charge-independent and charge-separated W production, and compared to two cutting-edge calculations evolved to next-to-next-to-leading order in quantum chromodynamics. In addition, this process strongly constrains fits of the Parton Distribution Functions of the proton; an analysis of the structure of the proton using several previous ATLAS measurements, including W boson production in association with jets at √s=8 TeV, will also be presented.

Abstract:

Abstract:

The Mu3e experiment aims to find or exclude the lepton flavour violating decay $\mu \rightarrow eee$ at branching fractions above $10^{-16}$. A first phase of the experiment using an existing beamline at the Paul Scherrer Institute (PSI) is designed to reach a single event sensitivity of $2\cdot 10^{-15}$. We present an overview of all aspects of the technical design and expected performance of the phase~I Mu3e detector. The high rate of up to $10^{8}$ muon decays per second and the low momenta of the decay electrons and positrons pose a unique set of challenges, which we tackle using an ultra thin tracking detector based on high-voltage monolithic active pixel sensors combined with scintillating fibres and tiles for precise timing measurements.