| The Standard Model is the currently accepted model of elementary particle interactions as determined by many years of particle physics experiments. It is both highly predictive and successful in its predictions, which is one of the great achievements of the last half century of science. However, there are indications that the Standard Model is incomplete with the main evidence being the so called hierarchy problem. In general terms, the hierarchy problem suggests that the parameters of the Standard Model have to be severely fine tuned in order to describe the real world. Motivated by the naturalness issues the hierarchy problem provokes, many theories of beyond the Standard Model physics have been proposed that alleviate the fine tuning. Continuing this approach, recently a new scenario called "Little Higgs" theories has been suggested as a viable alternative to the Standard Model, which has some novel features compared to the other competitive theories. The fact that these Little Higgs models may describe the real world is interesting and motivates their study.; In the first chapter, an introduction describing in elementary terms what the hierarchy problem is and how Little Higgs theories are a solution to this problem is presented. After this short introduction, two topics in Little Higgs theories are analyzed. The first subject is determining if Little Higgs theories are consistent with the precision experiments performed to date. This is covered in chapters two and three, where two different Little Higgs models with approximate custodial SU(2) symmetry are presented. This symmetry increases the range of parameter space where these Little Higgs theories are both consistent with precision tests and are natural under the sense of the hierarchy problem. Thus, these theories are viable candidates for beyond the Standard Model physics. The fourth chapter is on the second subject, which is analyzing the constraints that unitarity places upon the Little Higgs theories. The important consequence is that requirements for unitarity suggest that there is new physics in these theories at lower scales than previously expected. This can have implications for future experiments, as this physics can consist of particles light enough to be produced at the next generation of particle accelerators. |
