VLQs

VLQs, or vector-like quarks, are hypothetical fermions that extend the Standard Model by having left- and right-handed components that transform identically under the electroweak gauge group. This feature allows them to acquire gauge-invariant bare masses independent of the Higgs mechanism. Vector-like quarks often appear in theories beyond the Standard Model, including composite Higgs models, extra-dimensional models, and little Higgs scenarios, where they can help address questions related to electroweak symmetry breaking and the hierarchy problem.

VLQs come in various electric charges and SU(2) multiplets, including T-like quarks with charge 2/3, B-like quarks with charge −1/3, and exotic states such as X with charge 5/3 or Y with charge −4/3. They typically mix with Standard Model quarks through Yukawa-like couplings, leading to shifts in Higgs and gauge couplings and, in some realizations, potential flavor-changing neutral currents. The mass scale is largely set by a bare parameter M, and mixing with SM quarks is often constrained by precision measurements.

Phenomenology at colliders focuses on production and decay patterns. VLQs can be produced in pairs via QCD interactions, or singly through electroweak processes if mixing is sizable. Their decays reflect their charges: T-like quarks decay to Wb, Zt, or Ht; B-like to Wt, Zb, or Hb; X to Wt; and Y to Wb. Branching ratios depend on model details.

Experimental searches at the Large Hadron Collider have established limits on VLQ masses and mixing angles, typically pushing exclusions into the TeV range under common branching assumptions. No confirmed discovery has been reported as of the mid-2020s.