Catching Dark Photons in the Sky: Looking for light vector particles using satellites

Abstract

The lack of signals in traditional dark matter (DM) searches beyond gravitational interactions provide an excellent reason to investigate weakly coupled and/or light dark sectors (DS). These receive motivation both from the theoretical and the phenomenological side with rich physics across many different scales. Depending on the exact composition of the DS the signals can show up at colliders, in astrophysical searches, and in specialised direct detection experiments, which differ significantly from traditional nuclear recoil searches. Furthermore, the low mass of these new particles opens up new opportunities for producing new physics in the lab beyond the typical high-energy accelerators required for detecting new heavy states. This thesis will focus specifically on a broad class of DSs, namely those containing a Dark Photon (DP) in the particle spectrum. The DP is a well-motivated candidate which can act as a mediator between the standard model (SM) and the DS but it is also known to provide a rich phenomenology by itself. This ranges from the oblique corrections to electroweak precision observables (EWPO) at high DP masses to the subtle impact of a light DP on electromagnetism (EM). This thesis will provide an introduction to the conventional coupling of the DP to the full electroweak theory via the so-called kinetic mixing. We will further discuss potential generalisations of this mixing, i.e. via the Z boson mass or the Stückelberg mechanism. We will show how a light DP decouples from observables at colliders and that the kinetic mixing to electromagnetism is the low-energy theory emerging from the mixing with the hypercharge boson. This mixing with the photon enables non-trivial effects of the DP in an SM plasma. We will use the well-known plasma effects to calculate the solar DP flux emerging from the plasma of solar electrons. In contrast to the majority of the literature, we will focus on the angular distribution of the solar DP flux and point out the advantage of applying this additional information to helioscope searches for DPs. Using data from Hinode XRT, a solar x-ray telescope, we demonstrate that the sensitivity boost can be significant. This observation is interesting in light of future helioscopes, which is briefly discussed in this thesis as well. The second part of this thesis will then be focussing on the dark matter puzzle and the potential role of the DP as a DM candidate. After providing a brief introduction to the topic, we consider the LISA Path nder (LPF) mission as a direct detection experiment for ultralight DPDM. In the spirit of a "no-go theorem", we provide reasons why the direct detection of kinetically mixed DPs using LPF is impossible. This result is also valid for other direct detection setups with only a few caveats. However, we demonstrate that LPF is an interesting instrument for investigating the signal of "general DPs", i.e. DPs gauged under global SM symmetries such as B − L . We point out that the auxiliary channels of LPF provide a powerful tool to look for the DM signal, emphasising why it is expected to be better than the main channel. In addition, potential follow-up projects are discussed.