Best Paper Awards 2023New constraints on extended Higgs sectors from the trilinear Higgs coupling

18 April 2023

Photo: J. Braathen

The article by theoretical physicists H. Bahl (University of Chicago), J. Braathen (DESY), and G. Weiglein (DESY and University of Hamburg) presents a powerful new method to probe theories of New Physics. Particle Physics aims to understand the fundamental building blocks of Nature – elementary particles – and the forces between them. Some of the most pressing challenges in the field include understanding the early Universe and the origin of masses of elementary particles. These questions are left unanswered by our current best model (the "Standard Model") and mean that some New Physics must exist to address these questions.

Particle physicists are always on the lookout to find ways to probe this New Physics. A key task is to develop theories that are both consistent theoretically and viable in the light of available experimental data. In this context, studying the properties of the newly discovered Higgs boson is particularly important. Indeed, the Higgs boson is unique as the only known subatomic particle of spin 0. It also relates to many – if not most – of the open problems of Particle Physics. In particular, while it is understood that the Higgs boson is at the origin of the generation of elementary particles, the exact way this happens remains unknown. The Higgs mechanism is essential in describing the earliest times of the Universe’s history. The precise dynamics of this mechanism can have profound implications, for example, to understand why the Universe contains more matter than antimatter – and why we exist.

The properties of the Higgs boson allow learning about the dynamics of the mass generation mechanism. This is especially true for the Higgs potential, the function that describes the energy of the vacuum for a given configuration of the Higgs field. The key physical quantity is the coupling describing the self-interaction of three Higgs bosons, directly controlling the behavior of the Higgs potential. This quantity is called trilinear Higgs coupling.

This paper investigates the trilinear Higgs coupling in models with extended Higgs sectors – as a specific example, in a model with a second Higgs doublet. The calculations show that significant enhancements compared to the Standard Model prediction for this coupling are possible for unconstrained choices of the model parameters. Furthermore, the paper shows that comparing the prediction for the trilinear Higgs coupling with the latest experimental bounds on this coupling provides a novel and powerful probe of theories with extended Higgs sectors. This method offers a new way to assess sensitivity to New Physics beyond what can be obtained with state-of-the-art theoretical constraints or the present experimental results.

This work also highlights the importance of calculating the trilinear Higgs coupling to the highest possible level of precision, taking into account quantum effects. Future prospects for the experimental determination of the trilinear Higgs coupling promise significant improvements when this new method is used on data.