Bridging particle physics and cosmology: the Higgs boson as a window to the dark universe

• Applicant: Dr. Spyridon Argyropoulos
• Subject Area: Nuclear and Elementary Particle Physics, Quantum Mechanics, Relativity, Fields
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 433129798

The discovery of the Higgs boson constituted a turning point in the history of modern physics. It marked the completion of the Standard Model of particle physics, but more importantly, it opened up a new window to probe the big open questions in particle physics and cosmology: what is the nature of the non-luminous matter in the observable galaxies, why is the expansion of the universe accelerating and what made antimatter disappear from the universe? The answers to these questions are technically known as dark matter, dark energy and baryogenesis, and many models that attempt to explain them, predict that they are intimately related to the properties of the Higgs boson. The production of a Higgs boson in association with a Z boson (mediating the weak interactions) has only recently been observed and constitutes a unique probe of new physics that has not yet been fully explored. When the Z boson decays to neutrinos it produces a signature of missing energy, which is similar to the signature produced by the hypothesised weakly interacting particles that constitute dark matter and dark energy. Moreover in several models the production of dark matter and dark energy particles is either enhanced or proceeds solely via the interaction with the Higgs boson, therefore probing the associated production of a Higgs boson with missing energy will provide crucial information for understanding the physics of the dark universe.This proposal aims to set up a comprehensive search programme for electroweak baryogenesis, dark matter and, for the first time, dark energy particles produced in association with a Higgs boson, using the ATLAS experiment at the Large Hadron Collider at CERN. It will develop around two axes: direct searches, which are expected to lead to the tightest constraints on these models and indirect searches, through differential measurements of the production of a Higgs boson in association with a Z boson, which are expected to provide model-independent constraints. This proposal aims to cover the gap in the exploration of this signature and, in collaboration with theorists, combine the constraints from the LHC with the constraints from other dark matter and dark energy searches in a common parameter space. This would allow us to determine which models remain viable and build a roadmap for the High-Luminosity LHC phase.

• DFG Programme: Independent Junior Research Groups