Light Weakly Interacting Particles: Constraints and Connection to Dark Matter

The so far unknown particle nature of dark matter is a main motivation for extending the Standard Model of particle physics. A recently promoted approach to solving this puzzle is the concept of hidden sectors. Since the interactions of such sectors with the visible sector are very weak, so are the current experimental bounds. Hidden sectors might even contain sub-GeV scale particles that have so far escaped detection. In this thesis, we study the phenomenology of Weakly Interacting Slim Particles (WISPs) as well as their connection to dark matter in different Standard Model extensions. In the Next-to-Minimal Supersymmetric Standard Model (NMSSM), a light CPodd Higgs, arising from spontaneous breaking of approximate symmetries, represents an example of a WISP. Light gauge bosons of an extra U(1) symmetry in a hidden sector are other well motivated candidates for WISPs and called hidden photons. Such light hidden photons appear naturally in supersymmetry or string theory and might resolve the observed deviation in the muon anomalous magnetic moment from predictions. Moreover, scenarios in which hidden sector dark matter interacts via a light hidden photon with the visible sector exhibit appealing features in view of recent astrophysical anomalies. We study how the coupling of the CP-odd Higgs A0 to fermions can be constrained by current measurements for the case where the A0 is lighter than two muons. Analysing measurements of different rare and radiative meson decays, the muon anomalous magnetic moment as well as results from beam dump and reactor experiments, we severely constrain the CP-odd Higgs to be heavier than 210 MeV or to couple to fermions four orders of magnitude weaker than the Standard Model Higgs. These results apply more generally to the coupling of an axion-like particle to matter. Hidden photons can be constrained by experiments since they couple to charged Standard Model particles via kinetic mixing with the ordinary photon. We derive several constraints on the kinetic mixing for MeV-scale hidden photons from their production in past electron beam dump experiments. Including previously unconsidered experiments and taking into account the experimental acceptances, we exclude parts of the parameter space which had not been constrained by any similar study before. Additionally, we analyse different extensions of the Standard Model in which the hidden sector contains a light dark matter particle besides the hidden photon. For a minimal toy model and string-inspired supersymmetric hidden sector models with gravity mediation, we perform a parameter scan and compute the dark matter relic abundance and the scattering cross sections in direct detection experiments. We then compare the results of these computations to current experimental measurements. In this way, for the different models, we find viable dark matter candidates with potentially interesting signals in direct detection experiments. In summary, this work shows that WISPs, even though they only interact weakly with the Standard Model, can be probed by experiments. Moreover, hidden photons especially in connection to dark matter are found to exhibit interesting phenomenological features.