We review the important features of Little Higgs models, illuminating the mechanisms that generate a naturally light Higgs field while avoiding excessive fine-tuning. The full spectrum of the Littlest Higgs model is analyzed in light of the recent discovery of the Higgs boson mass. We find viable regions in parameter space that are consistent with the standard model. / text

The Standard model of particle physics provides a successful theory to understand the experimental results of the electroweak and strong interactions. However, it does not have a satisfactory explanation for the hierarchy problem. Many extensions of the Standard Model that solve the hierarchy problem result in new particles. We will study the phenomenology of vector-like fermions resulting in theories where the Higgs boson is typically a pseudo-Nambu-Goldstone boson. In these theories we study the case where a heavy fermion will be heavier than a heavy gluon, and then the channel of a heavy fermion decaying into a color octet is considered. We study this phenomenology at high energy colliders, both the LHC as well as future machines. / O modelo padrão de física de partículas fornece uma teoria bem-sucedida para entender os resultados experimentais das interações eletrofracas e fortes. No entanto, não tem uma explicação satisfatória para o problema de hierarquia. Muitas extensões do Modelo Padrão que resolvem o problema hierarquia resultam em novas partículas. Estudaremos a fenomenologia de férmions vetoriais resultando em teorias onde o bóson de Higgs é tipicamente um bóson pseudo-Nambu-Goldstone. Nessas teorias, estudaremos o caso em que um férmion pesado será mais pesado do que um glúon pesado, e então o canal de um férmion pesado decaindo em um octeto de cor é considerado. Estudamos esta fenomenologia em colisores de alta energia, tanto para o LHC quanto as futuras máquinas.

The Standard model of particle physics provides a successful theory to understand the experimental results of the electroweak and strong interactions. However, it does not have a satisfactory explanation for the hierarchy problem. Many extensions of the Standard Model that solve the hierarchy problem result in new particles. We will study the phenomenology of vector-like fermions resulting in theories where the Higgs boson is typically a pseudo-Nambu-Goldstone boson. In these theories we study the case where a heavy fermion will be heavier than a heavy gluon, and then the channel of a heavy fermion decaying into a color octet is considered. We study this phenomenology at high energy colliders, both the LHC as well as future machines. / O modelo padrão de física de partículas fornece uma teoria bem-sucedida para entender os resultados experimentais das interações eletrofracas e fortes. No entanto, não tem uma explicação satisfatória para o problema de hierarquia. Muitas extensões do Modelo Padrão que resolvem o problema hierarquia resultam em novas partículas. Estudaremos a fenomenologia de férmions vetoriais resultando em teorias onde o bóson de Higgs é tipicamente um bóson pseudo-Nambu-Goldstone. Nessas teorias, estudaremos o caso em que um férmion pesado será mais pesado do que um glúon pesado, e então o canal de um férmion pesado decaindo em um octeto de cor é considerado. Estudamos esta fenomenologia em colisores de alta energia, tanto para o LHC quanto as futuras máquinas.

The littlest Higgs model is one of the most economical solution to the hierarchy problem of the standard model. It predicts existence of new gauge vectors and also new scalars, neutral and charged. The littlest Higgs model predicts the existence of new scalars beside a scalar that can be assigned as Higgs scalar of the standard model. In this thesis, the production of scalars in $e^+e^-$ colliders is studied. The scalar productions associated with standard model Higgs boson are also analyzed. The effects of the parameters of the littlest Higgs model to these processes are examined in detail. The collider phenomenology of the littlest Higgs model is strongly dependant on the free parameters of the model, which are the mixing angles $s,s&#039 / $ and the symmetry breaking scale $f$. The parameters of the model are strongly restricted when the fermions are charged under only one $U(1)$ subgroup. In this thesis, by charging fermions under two $U(1)$ subgroups, the constraints on the symmetry braking scale and the mixing angles are relaxed. In the littlest Higgs model, the existence of charged heavy scalars also displays an interesting feature. By writing a Majorano like term in the Yukawa Lagrangian, these heavy charged scalars are allowed to decay in to lepton pairs, violating lepton number and flavor. In this thesis, the leptonic final states and also the lepton flavor and number violating final signals are also analyzed. As a result of these thesis, it is predicted that the scalar production will be in the reach for a $sqrt{S}=2TeV$ $e^+ e^-$ collider, giving significant number of lepton flavor violating signals depending on the Yukawa couplings of the flavor violating term.

Orientador: Alex Gomes Dias / Dissertação (mestrado) - Universidade Federal do ABC, Programa de Pós-Graduação em Física, 2009