Wing Sheung Chan

Chapter 1. The Standard Model and lepton flavour violation “This isn’t the kind of story where understanding makes you smart, or not understanding makes you dumb.” — CLAMP, “Y¯uko Ichihara”, xxxHOLiC Before we dive into the technical details of the search for lepton-flavour-violating Z → `τ decays, let us first motivate ourselves by reviewing things that “we know we know” – the Standard Model, that “we know we don’t know” – unsolved mysteries of the Standard Model, and that “we don’t know if we know” – beyond-the-Standard-Model theories, in relation to lepton flavour violation. In this chapter, the Standard Model will first be introduced in a conceptual and intuitive way and with a focus on the electroweak sector. We shall then discuss flavour violation in the quark and neutrino sectors of the SM, and after that, motivate ourselves to search for charged lepton flavour violation. Finally, we will discuss what makes the Z → `τ decay specifically the chosen search channel for this thesis. 1.1. Units and conventions Before the chapter actually begins, let us make some quick remarks on the units and conventions to avoid possible confusions. Throughout this thesis, we will be using a conventional system of natural units, one that is commonly adopted in particle physics, to simplify expressions. In this system, velocities are expressed in units of the speed of light c = 299 792 458 m s − 1 (1.1) 3