Collider Phenomenology of Extra Dimensions

Abstract

In recent years there has been much interest in the possibility that there exist more spacetime dimensions than the usual four. Models of particle physics beyond the Standard Model that incorporate these extra dimensions can solve the gauge hierarchy problem and explain why the fermion masses a spread over many orders of magnitude. In this thesis we explore several possibilities for models with extra dimensions. First we examine constraints on the proposal of Arkani-Hamed and Schmaltz that the Standard Model fermions are localized to different positions in an extra dimension, thereby generating the hierarchy in fermion masses. We find strong constraints on the compactification scale of such models arising from flavor-changing neutral currents. Next we investigate the phenomenology of the Randall-Sundrum model, where the hierarchy between the electroweak and Planck scales is generated by the warping in a five-dimensional anti-de Sitter space. In particular, we investigate the ''Higgsless'' model of electroweak symmetry breaking due to Csaki et. al., where the Higgs has been decoupled from the spectrum by taking its vacuum expectation value to infinity. We find that this model produces many distinctive features at the LHC. However, we also find that it is strongly constrained by precision electroweak observables and the requirement that gauge-boson scattering be perturbative. We then examine the model with a finite vacuum expectation value, and find that there are observable shifts to the Higgs scalar properties. Finally, in the original large extra dimension scenario of Arkani-Hamed, Dimopoulos, and Dvali, the hierarchy problem is solved by allowing gravity to propagate in a large extra dimensional volume, while the Standard Model fields are confined to 4 dimensions. We consider the case where there are a large number of extra dimensions (n {approx} 20). This model can solve the hierarchy problem without introducing a exponentially large radii for the extra dimensions, and represents a scenario that is difficult to obtain in string theory. We show that, if this scenario holds, the number of dimensions can be constrained to be larger than the number predicted by critical string theory. Searching for signals of many dimensions is then an important test of whether string theory is a good description of quantum gravity.