Flavorchanging processes of quarks and leptons as indirect probes of physics beyond the Standard Model

Abstract

The deepest knowledge of the nature behavior is today represented by Standard Model (SM) of particle physics. This quantum eld theory is brie y presented in Chapter 1, where we discuss its foundations in terms of particles and gauge symmetries. In particular we focus on the scalar and

avor sector, that are the main concern of this Thesis. We show that the elegance of the theory seems to be completely lost when we consider the ultra-high sensitivity to the short-distance e ects in the scalar mass term (hierarchy problem) or when we try to explain the peculiar patterns of masses and mixings that we observe in quarks and leptons ( avor puzzle). Anyway if the hierarchy problem seems to indicate that some unknown completion of the SM should be at the TeV scale, the experimental data agree with high precision with the SM. For this reason we know that if some New Physics (NP) is close it must be highly non-generic, specially in its avor sector ( avor problem). After quantifying this considerations in Chapter 2, we present a possible solution to this problem (at least from the avor point of view), that is the Minimal Flavor Violation hypothesis. In Section 2.3 we present one of the most prominent candidates for a theory beyond the SM, that is the supersymmetry. We again discuss with particular attention the Higgs and avor sector of the Minimal Supersymmetry Standard Model. In particular we show that within the minimal framework of this theory the recent LHC results are pushing higher and higher the scale of the supersymmetric particles, stepping away from the naturalness expectations. Nevertheless, some new ideas able to explain what has been observed so far have emerged. For example the squarks of the rst two families can be signi cantly heavier than the third generations (split-family SUSY), explaining the fact that we haven't seen any supersymmetric particle and keeping the theory natural at the same time. This possibility is analyzed in Chapter 3, where we present a speci c avor model compatible with such a scenario and based on a U(2)3 avor symmetry. We show that this symmetry is not only able to give the desired suppression to Flavor Changing Neutral Currents, but it can also provide some insight into the SM avor puzzle. However the U(2)3 model is able to describe only the quark sector, while in a complete theory we need to enlarge the description also to leptons. This goal is achieved in [1] and presented in Section 3.2. Starting from the maximal U(3)5 symmetry we show how it is possible to obtain a two steps breaking leading to O(3) in the neutral sector and to U(2)5 in the charged sector, to be able to provide a good description of the lepton sector and keeping the (s)quark as in the original U(2)3 model. We also calculate the Lepton Flavor Violation processes, testing the goodness of the model in this respect. The same model is analyzed also in Section 3.3, where we assume the U(2)3 avor symmetry to be broken at a very high scale, instead of being directly applied at the electroweak scale [2]. We present the Renormalization Group Equations e ects on the low-energy parameter space. For example we check under which conditions it is possible to obtain a splitted squark spectrum and if the typical U(2)3 mixing properties are preserved by the running. We also study possible deviations from the minimal breaking patter of the avor symmetry and the e ects of some usually neglected F = 2 operators. Beside supersymmetry, other New Physics theories have been proposed in order to solve (at least in part) the problems of the SM. In the second part of this Thesis we take a more general approach: we analyze in a model-independent way a few phenomenological consequences of pos- sible deviations from the SM Higgs sector. In fact almost all the SM problematic aspects are in its scalar sector (hierarchy problem, avor puzzle, vacuum stability, neutrino masses and also cos- mological constant problem) and for this reason a motivated NP Higgs sector usually introduces several new features not present in the SM. Thus, the recent LHC discovery of a new scalar neutral particle gives us the important oppor- tunity to probe the SM and its possible completions in one of their crucial aspects. In Section 4.1 we present a general analysis where we i) analyze the indirect constraints on possible avour- violating couplings of the Higgs set by low-energy avour-changing transitions, ii) analyze the consequences of these constraints on possible avour-violating decays of the Higgs boson [3]. In particular we show which are the avor-changing decays that could be observed with the LHC sensitivity. In Section 4.2 and 4.3 instead, we focus on a class of theories beyond the SM, that is the 2 Higgs Doublets Model. In this framework we analyze the B-physics phenomenology and in particular the decay B ! that seems to show a slight tension with the SM expectation [4]. We present under which conditions it is possible to obtain an enhancement in the decay rate assuming Minimal Flavor Violation and considering all the other avor processes relevant at large tan . We also show a preliminary study, where we compare the discovery potential of the processes B ! and Bs ! , in view of the experimental prospects of LCHb and a possible superB machine [5].