In the Standard Model (SM), the electromagnetic and weak interactions arise from a SU(2)xU(1) gauge symmetry that is spontaneously broken by the Englert-Brout-Higgs mechanism. Measurements at collider experiments are so far consistent with its predictions. However, the SM is believed to be only a low-energy approximation of a more fundamental theory due to several unanswered questions. For example, it cannot explain the matter-antimatter asymmetry in the universe and the origin of dark matter. When the SM is extrapolated to high energies, fine-tuning is required due to divergent corrections to the Higgs boson self-energy. Solutions to this so-called hierarchy problem are proposed in several beyond-the-Standard Model (BSM) theories, which can be considered a first step toward a more fundamental theory of particle physics.

Since a large contribution to the fine-tuning originates from top-quark loop corrections, the hierarchy problem can be reduced in models predicting top-quark partners that mitigate the SM top quark's contribution: while a scalar top-quark partner appears in supersymmetry as the bosonic superpartner of the top quark, fermionic top-quark partners appear in theories with a new broken global symmetry, in which the Higgs boson is interpreted as a pseudo Nambu-Goldstone boson as, for instance, in Composite Higgs models. In these models, the new symmetry corresponds to a new strong interaction, whose bound states include vector-like quarks (VLQ). These are color-triplet spin-1/2 fermions, but in contrast to the chiral SM quarks their left- and right-handed components have the same properties under the electroweak symmetry group. If the Higgs sector is unchanged, gauge invariance implies that only a limited set of possibilities exists for the quantum numbers of the VLQs and thus their electric charge could only be 2/3e (T quark), -1/3e (B quark), 5/3e (X quark) or -4/3e (Y quark), with such new quarks appearing in electroweak singlets, doublets and triplets.

In this seminar the different production and decay modes of the VLQs will be discussed, as well as the experimental signatures in hadron colliders. The status of the searches for vector-like quarks by the ATLAS and CMS collaborations will be reviewed. In particular, the experimental techniques and the explored topologies, as well as the statistical and phenomenological interpretations of the obtained results, will be discussed. The assumptions beyond the experimental searches and possible improvements for the future will be addressed.

Since a large contribution to the fine-tuning originates from top-quark loop corrections, the hierarchy problem can be reduced in models predicting top-quark partners that mitigate the SM top quark's contribution: while a scalar top-quark partner appears in supersymmetry as the bosonic superpartner of the top quark, fermionic top-quark partners appear in theories with a new broken global symmetry, in which the Higgs boson is interpreted as a pseudo Nambu-Goldstone boson as, for instance, in Composite Higgs models. In these models, the new symmetry corresponds to a new strong interaction, whose bound states include vector-like quarks (VLQ). These are color-triplet spin-1/2 fermions, but in contrast to the chiral SM quarks their left- and right-handed components have the same properties under the electroweak symmetry group. If the Higgs sector is unchanged, gauge invariance implies that only a limited set of possibilities exists for the quantum numbers of the VLQs and thus their electric charge could only be 2/3e (T quark), -1/3e (B quark), 5/3e (X quark) or -4/3e (Y quark), with such new quarks appearing in electroweak singlets, doublets and triplets.

In this seminar the different production and decay modes of the VLQs will be discussed, as well as the experimental signatures in hadron colliders. The status of the searches for vector-like quarks by the ATLAS and CMS collaborations will be reviewed. In particular, the experimental techniques and the explored topologies, as well as the statistical and phenomenological interpretations of the obtained results, will be discussed. The assumptions beyond the experimental searches and possible improvements for the future will be addressed.

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With support from FCT through project UIDB/00777/2020

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