Expecting the unexpected: going beyond the standard searches for vector-like quarks at the ATLAS experiment at CERN

DOMAIN: Particle and Astroparticle Physics and associated scientific domains


CO-SUPERVISOR: Miguel Crispim Romão

HOST INSTITUTION: Laboratório de Instrumentação e Física Experimental de Partículas

DEGREE INSTITUTION: Universidade do Minho


A cornerstone of the Standard Model (SM) of Particle Physics is the formulation of the electroweak interactions as arising from a spontaneously broken gauge symmetry. Experiments over the past four decades have confirmed this hypothesis with precision, most notably the LEP and SLC collider programs. The ATLAS and CMS collaborations reported in 2012 observations of a new particle produced at the CERN Large Hadron Collider (LHC) possessing properties so far consistent with those predicted for the SM Higgs boson. It should be noted, nonetheless, that the exact nature of the symmetry-breaking mechanism is not yet determined. Furthermore, regardless of the many experimental validations of the SM, it is known that it cannot be regarded as a final theory, given the many problems it fails to solve, namely the hierarchy problem, the lack of a dark matter candidate, matter-antimatter asymmetry, and others. Attempting to solve these problems, many beyond the SM theories were built. Vector-like quarks (VLQ) are featured in some of these models, as spin 1/2 fermions, color triplets with the same left and right quantum numbers, mixing with SM quarks. Both the ATLAS and CMS experiments have developed a comprehensive search program for vector-like quarks. Nonetheless, and despite a significant effort to present general results, most of these analysis made important assumptions on the production and decay modes of the vector-like quarks and thus missing possible alternative signatures, which can have strong phenomenological motivations. The current proposal foresees the development and implementation of a search for vector-like quarks which can be produced in pairs via a heavy gluon or decay into new scalars. Given the multitude of final states that vector-like quark production can have, the search strategy will rely on the use of deep learning. A complete experimental analysis, considering a detailed study of the modelling of backgrounds and sources of uncertainties will be developed. The phenomenological consequences of the experimental results will be also be studied, in close collaboration with the Phenomenology group at LIP. The current proposal will be integrated in the Portuguese ATLAS team and collaboration with several international groups is foreseen.