DARK PHOTON PHYSICS: A COMPREHENSIVE REVIEW OF THEORETICAL MODELS, DARK MATTER CONNECTIONS, EXPERIMENTAL SEARCHES, AND FUTURE PROSPECTS

Abstract

The nature of dark matter remains one of the most significant unresolved problems in modern particle physics and cosmology. Astronomical observations indicate that only about 5% of the Universe consists of ordinary matter. Approximately 27% is composed of dark matter, while nearly 68% is dark energy. Among the many theoretical candidates proposed to explain dark matter, the dark photon has emerged as one of the most promising and experimentally accessible possibilities. The dark photon is a hypothetical gauge boson associated with an additional hidden  gauge symmetry. Unlike the ordinary photon, the dark photon can possess a small non-zero mass and interact weakly with visible matter through a mechanism known as kinetic mixing. This interaction provides a natural portal between the visible sector and a hidden dark sector, enabling potential experimental detection while remaining consistent with current observational constraints. This article presents a comprehensive overview of the different aspects of dark photons, including their theoretical motivation, mathematical formulation, production mechanisms, decay channels, cosmological implications, astrophysical significance, and experimental searches. The theoretical framework is developed by introducing an extended gauge symmetry with kinetic mixing between the electromagnetic field and the hidden gauge field. The dependence of dark photon phenomenology on its mass and kinetic mixing parameter. Various production mechanisms, including thermal production, freeze-in processes, meson decays, bremsstrahlung, and collider production, are examined. The possible decay modes into Standard Model particles and hidden-sector particles are also analyzed.  The cosmological consequences of dark photons are explored in the context of the early Universe, Big Bang nucleosynthesis, the cosmic microwave background, and structure formation. Their possible role as mediators of dark matter interactions and their influence on galaxy dynamics are also discussed. Furthermore, current and future experimental efforts, including collider experiments, beam-dump facilities, fixed-target experiments, precision atomic measurements, and direct detection searches, are reviewed. Constraints on the dark photon parameter space obtained from laboratory experiments and astrophysical observations are summarized. The study highlights how dark photons provide a compelling framework that connects particle physics, cosmology, and astrophysics while offering experimentally testable predictions. Continued theoretical developments and next-generation experiments are expected to significantly improve sensitivity to dark photon signatures, potentially revealing new fundamental interactions beyond the Standard Model. The dark photon therefore remains a leading candidate in the search for new physics and may play a crucial role in understanding the composition and evolution of the Universe.