Exploring dark sectors in particle physics

Abstract

Dark matter provides one of the strongest pieces of evidence for physics beyond the Standard Model and motivates a wide range of dark sector scenarios. In this thesis, we investigate theoretical consistency conditions of gauge symmetry extensions and phenomenological signatures of two concrete dark matter models. We first study extensions of the Standard Model by an additional U(1) gauge symmetry with a new massive gauge boson Z′, focusing on constraints arising from anomaly cancellation. We compute the axial–axial–vector triangle diagram inducing an effective Z–Z′–γ vertex and analyse its implications for the decay Z → Z′γ. We consider two representative examples, U(1)B−L and U(1)B, and show that the additional fermionic states that have to be introduced in the second case can lead to non-trivial effects such as non-decoupling. We derive constraints on the parameter space from Z → Z′γ and relate our results to effective field theory descriptions. We then study the collider phenomenology of inelastic dark matter, where a small mass splitting between dark sector states suppresses direct detection signals but can lead to distinctive signatures at the Large Hadron Collider. Using simplified models with different mediator types, we recast an ATLAS monojet search and obtain exclusion limits, finding the strongest constraints for vector and axial-vector interactions. We further examine displaced vertex signatures and show that observable effects are mainly restricted to pseudoscalar mediators due to competing requirements on decay kinematics and lifetimes. We identify the key parameters controlling collider sensitivity and discuss implications for future searches. Finally, we investigate a leptophilic dark matter model in which interactions with the Standard Model are mediated by a charged t-channel particle. In this scenario, dark matter annihilation can produce gamma-ray features via virtual internal bremsstrahlung. We show that the same interactions generate electromagnetic form factors at loop level, leading to anapole and dipole moments that can give rise to observable direct detection signals. The resulting constraints are strongest in the parameter region that enhances the gamma-ray feature, highlighting the complementarity between indirect and direct detection probes. Overall, this work demonstrates the interplay between theoretical consistency and phenomenological signatures in dark sector models and shows how combining different experimental approaches significantly constrains physics beyond the Standard Model.