Dynamics of the Top Quark Beyond the Standard Model.

Abstract

The top quark, the heaviest fermion that has been observed, is an important particle in understanding the origin of electroweak symmetry breaking. This thesis is about various collider phenomenologies of the top quark and new physics that puts special emphasis on the top quark.

Firstly, the forward-backward asymmetry of the top quark is discussed, as it is measured at the Tevatron to be $2 sim 3.5 sigma$ larger than the Standard Model prediction. I argue that if the deviation is the effect of new physics, $t$-channel physics is promising candidate responsible for it. I detail model building based on a new abelian gauge symmetry. These physics ideas can be discovered at hadron colliders through precise measurements of the top pair cross section, especially in the high invariant mass region, as well as the production of like-sign top pairs, which can be observed as events containing like-sign dileptons.

Second, it is argued that the most important discovery mode of a warped extra dimension can be production of four top quarks. This is especially true if the bulk profiles of light quarks are almost flat allowing a relatively light Kaluza-Klein scale. This ansatz is motivated from the results of electroweak precision tests and flavor physics. Depending on Kaluza-Klein gluon couplings to third generation fermions, the like-sign dilepton or the single lepton topology of four top quarks can be the best collider signature.

Finally, in supersymmetric theories, the superpartner of the top quark -- the stop -- is often the lightest scalar super-partner. Heavy scalars of the first two generations suppress several generically large contributions to well-measured observables including the electric dipole moment. Given the possibility of the LHC improving its probing sensitivity, the discovery reach of the LHC and precision measurements of dipole moments are compared within supersymmetry frameworks having light stops. Although dipole moments are better probes within the near future, the LHC can provide another interesting independent opportunity for discovery.