Vector-Like Quarks and Other Physics Beyond the Standard Model

The Standard Model (SM) was first proposed more than 50 years ago and it is still one of the most successful theories in science. There are, however, some phenomena that the SM cannot explain, such as dark matter or the matter anti-matter asymmetry. Thus, it is the job of theoretical particle physicists to propose models which complement the SM, explaining some phenomena that it cannot.

One such class of models are models with vector-like quarks (VLQs). VLQs are fermions which, like the SM quarks, are triplets of SU(3) but whose left- and right-handed components transform in the same way under SU(2). On the one hand, they are motivated from the top-down approach since they appear in some Grand Unified models, such as E_6, and they also appear in some extra dimension models, such as the Randall and Sundrum warped geometry models. On the other hand, they are also motivated from the bottom-up, since they can explain deviations from unitarity of the CKM matrix and they can give new sources of CP violation.

They can appear in seven different types of gauge covariant multiplets. Out of those, the singlets with hypercharges 2/3 or -1/3 and the doublets with hypercharge 1/6 are the representations which contain only fields with the same electric charge as the SM quarks, making them the simplest VLQ extensions. We have studied these extensions.

There is a bigger class of models beyond the SM (which include models with VLQs), which are the models which have the same electroweak gauge group as the SM, i.e., SU(2) x U(1). For this class of models, the New Physics (NP) effects can be parametrized by a set of observables called oblique parameters. We have computed the oblique parameters in different general NP frameworks, namely in extensions of the SM with extra fermions (which include extensions with VLQs) and in extensions of the SM with extra scalars.