Among the many beyond-the-Standard-Model (BSM) scenarios that have been proposed and investigated during the last thirty years, vector-like fermion extensions of the Standard Model seem particularly interesting, as they present features relevant both from the theoretical and phenomenological point of view. They can be utilized to construct such UV completions of the Standard Model that present either asymptotic freedom or asymptotic safety. They can explain some of the intriguing anomalies that have been recently observed in rare B-meson decays. Finally, there is a wide set of experimental measurements that can, directly or/and indirectly, detect their presence.

Research objectives

The underlying goal of the project is to construct and analyze vector-like models that can explain the recently observed flavor anomalies and in the same time to provide a viable UV completion of the Standard Model. The milestone research objectives include:

  • To analyze phenomenological properties of the BSM scenarios with vector-like fermions.
  • To study theoretical properties of various vector-like extensions of the Standard Model.
  • To work out, in collaboration with experimentalists, the best research strategies to look for the vector-like fermions.
  • To develop numerical tools that will be used in global analyses of the vector-like BSM extensions.


The project has been carried out for five years and has entered its one-year extension. Below we summarize its most important results.

  • BSM extensions with two distinct vector-like representations, two distinct scalars, or one scalar and one vector-like fermion, can allow for a precise unification of the gauge couplings. Interestingly, the required mass spectra are very hierarchical. That opens an enticing possibility to experimentally probe such scenarios with a variety of collider searches up to the masses in the multi-TeV regime. In that respect, long-lived particle searches are particularly effective.
    More info: journal and preprint publications 1, 2, talks 1, 2, 3.
  • Vector-like fermions can play a role in explaining flavor anomalies in b→s tranistions. For example, through the mixing with the Standard Model fermions they can generate flavor-changing couplings with a hypothetical new gauge boson, Z'.
    More info: journal and preprint publications 1, 2, 3, 4, talks 1, 2, 3.
  • The BSM extensions with vector-like fermions can be UV-completed in the framework of asymptotic safety. A virtue of such a construction is that it can give specific predictions for the parameters of the lagrangian, which otherwise would remian unconstarined. We analyzed a set of asymptotically safe BSM VL models that can explain the muon (g-2) anomaly and found that only a few of them are valid.
    More info: journal publications 1, 2, 3, 4, 5, talks 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11.
  • Vector-like fermions are also present by construction in supersymmetry. Gauginos, fermionic partners of the gauge bosons, and higgsinos, partners of the Higgs bosons, have exactely this property. Among them, long-lived charginos are of particular ineterst as their pair-production at the LHC can provide a way of probing the higgsinos as DM candidates.
    More info: preprint publications 1, 2, talks 1.
  • Another interesting application of vector-like fermions is to dynamically generate masses and mixings of the Standard Model fermions. This can be achieved via mixing between the thrid generation quarks and leptons with the vector-like fermions, or via a seesaw-like mechanism for the second and first generations.
    More info: preprint publications 1, talks 1.