Lepton Flavour Violation and. Dark Matter Phenomenology (original) (raw)

GENERAL INTRODUCTION AND MOTIVATIONS One of the open questions in particle physics nowadays is the so-called flavour puzzle: why there is a hierarchical structure of fermion masses and mixings, and why there are two replicas of the lightest fermions? The leptonic sector presents even more challenging features than the quark sector, where the mixings between flavour eigenstates show an approximately perturbative texture. Indeed, in the neutrino sector the values of the mixing angles appear 'randomly' distributed, and one of the mixing angles is close to maximal. On the other hand, the masses of neutrinos are believed to be below ∼ 0.3 eV, i.e. six orders of magnitude smaller than the lightest charged fermion (the electron). This should be compared with the ∼ five orders of magnitude expanded by the masses of the nine charged fermions (from the electron to the top mass), more or less equally distributed in between. Certainly, the Standard Model (SM) can be trivially extended to accommodate neutrino masses. However, the previous huge gap between the neutrinos and the rest of SM particles has posed a strong motivation to develop theoretical models that could explain it. One of the most popular theoretical frameworks to accommodate such small neutrino masses is the socalled Seesaw mechanism. The idea is that right-handed (RH) neutrinos, which are singlets under the SM gauge group, can have large Majorana masses (as they are not controlled by the electroweak-breaking scale). Then the lightest (approximately pure left-handed) neutrinos get masses suppressed by the ratio of the Higgs VEV and the right-handed Majorana mass, thus becoming extremely light. The Seesaw scenario can be easily formulated within a supersymmetric framework. Indeed, this is highly motivated by the fact that the massive right-handed neutrinos introduce large (logarithmic) corrections to the Higgs mass, which worsens the notorious Hierarchy-Problem of the Standard Model. On the other hand, the neutrino Yukawa couplings must present an off-diagonal structure (to generate the neutrino mixings), which in turn induces off-diagonal entries in the slepton matrices. The latter may potentially trigger processes which violate Lepton flavour, for example µ → eγ.