Precision Calculations in the Higgs Sector - Paving the Way to the New Physics Landscape

• Applicant: Professorin Dr. Milada Margarete Mühlleitner
• Subject Area: Nuclear and Elementary Particle Physics, Quantum Mechanics, Relativity, Fields
• Term: from 2016 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 318580798

The Standard Model (SM) of particle physics summarizes our today's knowledge about the constituents of matter and their fundamental interactions. One of its main ingredients is the Higgs mechanism for the generation of particle masses. Although consistent at present with the most precise measurements, the SM suffers from flaws which call for New Physics (NP) extensions. These entail new particles. The search for NP and the exploration of the Higgs mechanism belong to the main tasks of the Large Hadron Collider (LHC). The particle associated with the Higgs mechanism was discovered by the LHC experiments almost fifty years after its postulation. New Physics, however, has not been discovered so far, apart from possible hints, that need more experimental data for their clarification. The Higgs boson has itself become a tool for the exploration of NP. The latter can show up in deviations of the Higgs couplings to the remaining SM particles, which would lead to non-standard production and decay rates. The present accuracy in the Higgs rates leaves room for interpretations within NP models. In order to properly interpret the rates, however, and to be able to draw conclusions on the underlying model, predictions at the highest precision have to be provided by theory. These are then tested by fits to the experimental data. The plethora of NP models on the market calls for a systematic approach in their exploration. In the framework of this research proposal we will investigate in a bottom-up approach some of the simplest extensions of the SM Higgs sector, that are compatible with the present experimental and theoretical constraints: the two Higgs doublet model (2HDM), its simplest extension by a singlet field (N2HDM), the SM extended by a real and a complex singlet field (RxSM and CxSM) and the next-to-minimal supersymmetric model (NMSSM). The Higgs sectors of these models contain more than one Higgs boson, are accessible at the recently launched LHC Run 2 and are partly already taken into account in the experimental analyses. We will calculate the electroweak corrections to the Higgs decays of these models, which in contrast to the QCD corrections cannot be taken over from the SM. The expected sizes of the corrections will be relevant for the proper interpretation of the data at LHC Run 2 and indispensable for the determination of the Higgs properties and hence for pinning down the underlying model in the high-luminosity phase of the LHC and at future linear colliders. The achieved precision in the predictions for the various models and the application of the same methods will enable their consistent comparison with the data. The goal of the foreseen subsequent phenomenological analyses, in which we will take into account all relevant experimental and theoretical constraints, is the identification of observables and signatures for the discovery of the Higgs particles and the distinction of the models.

• DFG Programme: Research Grants