Theoretical and phenomenological investigations of models with vector-like leptons

• Applicant: Professor Dr. Dominik Stöckinger
• Subject Area: Nuclear and Elementary Particle Physics, Quantum Mechanics, Relativity, Fields
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 552845731

Two important open questions of elementary particle physics are the origin of elementary particle masses and the origin of the three generations of quarks and leptons. Both are related to the Higgs boson and its interactions. So far, all measured properties of the Higgs boson agree with the predictions of the Standard Model (SM). However, the SM itself does not provide full answers to the above questions, and it is still possible that properties such as the Higgs potential or the Yukawa sector giving masses to quarks and leptons actually are quite different from the prediction of the SM. For these reasons, experiments at the LHC aim to scrutinize such Higgs boson properties with high accuracy, with the possibility to discover deviations from the SM and thus new physics. At the same time, the Higgs/Yukawa sector is also scrutinized by a set of lower-energy experiments such as muon g-2, or experiments searching for lepton flavour violation. The interpretation of these experiments requires theoretical efforts to develop alternatives to the SM, to derive testable predictions and to understand how experiments are sensitive to specific alternative hypotheses. A large number of popular models modifies the number of Higgs bosons and thus the Higgs potential. In the present project we pursue a less well studied but equally motivated idea: we study a model with vector-like leptons (VLL), which modify the Yukawa sector and the three-generation structure of leptons. In the VLL model, the origin of lepton masses is strongly altered (in a way resembling the seesaw-mechanism for neutrinos), with dramatic consequences that could be observed at the LHC Higgs-decay measurements, at the Fermilab muon g-2 experiment or at forthcoming lepton-flavour-violation experiments. The project combines phenomenological and theoretical investigations. We begin with a detailed study of the renormalization of the VLL model. This study will be important because of several peculiar aspects of the VLL model, and it will generally lead to an improved understanding of the renormalization of beyond-the-SM scenarios. As an outcome we will define an on-shell renormalization scheme which allows to express observables as function of the physical fermion masses. Second, we will carry out a detailed investigation of the phenomenology of the VLL model, with a focus on observables measured at the LHC and at complementary low-energy experiments which are sensitive to the dramatic effects from the modified mass-generation mechanism. We will take into account on-shell renormalized higher-order effects, which are likely to be large and important. In addition to these two main lines of work we will also develop an effective-field-theory approach to the model, and we will publish a computer code for the numerical analysis of the VLL model.

• DFG Programme: Research Grants