Vector-like fermions as candidates for New Physics

Abstract

The Standard Model of particle physics, while remarkably successful, leaves several fundamental questions unanswered. Issues such as the origin of fermion masses, the hierarchy problem, and the absence of viable dark matter candidates suggest the presence of new physics beyond the Standard Model. Among the many proposed extensions, models incorporating vector-like fermions offer a compelling avenue for exploration. Their non-chiral nature permits interactions with the Standard Model fields without introducing gauge anomalies, enabling flexible phenomenological implementations. This thesis presents a comprehensive investigation of models featuring vector-like fermions from both phenomenological and theoretical perspectives. It begins with an analysis of a minimal extension of the Standard Model featuring additional scalars and vector-like fermions. This model offers a novel mechanism for the generation of Standard Model fermion masses, while also accommodating the observed anomaly in the muon's magnetic moment and remaining consistent with current collider constraints through dedicated parameter space scans. The work then transitions to a more theoretical investigation. First, it examines the renormalization group flow in a toy model of vector-like quarks and SU(N_C) gluons, extended to include infinitely many flavors and colors. Using functional renormalization group techniques, the ultraviolet behavior of this model is studied non-perturbatively, and the existence of interacting fixed points is explored. The presence of such a fixed point, which may provide a UV completion, is a welcome feature, however, a scenario with an infinite number of fields is not realistic. Another theory that exhibits an ultraviolet fixed point is quantum gravity. The thesis investigates how quantum gravity effects, through the assumption of asymptotic safety, can impact the parameter space of beyond the Standard Model scenarios. Specifically, the idea that ultraviolet fixed points emerge from gravity-induced corrections is used to predict viable configurations of vector-like fermions and new gauge bosons, linking low-energy observables to Planck-scale physics. Overall, the results underscore the versatility of vector-like fermions in beyond the Standard Model model building and highlight the complementary roles of phenomenological studies and non-perturbative methods in exploring physics beyond the Standard Model.

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BibTeX

@phdthesis{Rizzo:2025thesis,
  title = {Vector-like fermions as candidates for New Physics},
  author = {Rizzo, Daniele},
  year = {2025},
  school = {National Centre for Nuclear Research},
  url = {/papers/Daniele_Rizzo_PhD_Thesis.pdf}
}

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