Wing Sheung Chan

158 Summary • Why are the neutrino masses so much lighter than the other matter particles in the SM? (the neutrino mass problem) Unable to address these problems, the SM simply could not be a complete theory in describing Nature. Therefore, physicists have been trying to extend the SM to create the so-called beyond-the-Standard-Model (BSM) theories. There are many different ways that the SM can be extended, and in order to know whether we are going in the right direction, we need experimental data to confirm or constrain our many BSM theories. Figure S.1.: Elementary particles in the Standard Model of particle physics. Lepton flavour violation In the SM, there are three generations of matter particles. In particular, leptons (a class of particles in the SM) come in three different flavours: the electron ( e ), the muon ( µ ) and the tau ( τ ) lepton. According to the SM, the number of leptons in each flavour remains the same before and after any interactions. For example, a Z boson (a massive neutral particle) can decay into a muon and an antimuon ( Z → µµ ), but never into a muon and an anti- τ lepton ( Z → µτ ) (Figure S.2). Violation of this rule is known as lepton flavour violation. Any observation of lepton flavour violation would certainly constitute an exciting sign of BSM physics. There are many BSM theories that permit lepton flavour violation. These theories are usually motivated by the unsolved mysteries mentioned above. For example, theories with heavy neutrinos are able to explain the smallness of the SM neutrino masses, while