PhD defence by Christine Hartmann

A Quest for New Physics - Loop Calculations of the Higgs Decay to Two Photons in the Standard Model Effective Field Theory

Abstract: The discovery of a Higgs like boson in 2012 closed many chapters within particle physics, providing a completion of the standard model field content. As a result of this event, many new chapters have opened. These take the shape of unresolved issues within the standard model, as well as physics, which the standard model can not incorporate. Such open questions and insufficiencies of the standard model provide exciting hints of new physics, which can not be ignored.

The quest for new physics has merely begun. The second run of the particle accelerator at CERN, LHC, is expected to provide interesting new data in the very near future. Such data requires careful treatment, also from a theoretical perspective. 

New physics could manifest itself through the discovery of new particles. On the other hand, with new physics possibly being out of reach for present and future experiments, it could instead show up in the shape of deviations within existing interactions of the standard model. This thesis addresses the latter case through explicit calculations, incorporating new physics in the shape of higher dimensional operators. The standard model is taken to be an effective field theory and extended to include new interactions appearing in these additional terms of the Lagrangian. 

The full one-loop contributions to the Higgs decay to two photons, within this standard model effective field theory, are presented.

These contributions allow for deviations of the SM expectations on the order of one percent. Such deviations will have an impact already with data from Run II, which will reach a comparable level of precision. 

In conclusion, the results presented in this thesis are necessary to take into account to obtain a proper fit of the standard model effective field theory to experimental data. Following these main results, an outline of work in progress regarding the two-loop contributions will be given.