Light hidden sectors

A new generation of experiments, including the Large Hadron Collider, is leading particle physics into the data-driven era. This era may bring the discovery of new physics at higher energies than the particle masses of the Standard Model. An equally exciting possibility is the existence of new physics at low energies, such as the GeV scale, with the new physics very weakly coupled to the Standard Model. We focus on the second scenario in this thesis, and we address the question: how could the possible existence of a new, light, hidden sector of particles modify upcoming experimental results? In the first part of the thesis, we build a series of models where dark matter resides in a light hidden sector. We consider scenarios where dark matter annihilates into hidden sector particles, decays into hidden sector particles, or carries an asymmetry under a conserved quantum number of the hidden sector. Each of these cases has exciting consequences for the direct or indirect detection of dark matter. In the second part of the thesis, we consider how a light hidden sector can be discovered in a high-energy collider, such as the Large Hadron Collider. Standard Model particles can decay into highly-boosted hidden sector particles, that manifest themselves as collimated bunches of leptons, known as lepton jets. The discovery of an excess of these new objects would constitute a smoking-gun signature of the existence of a light hidden sector. Lepton jets are now being searched for at the Tevatron and Large Hadron Collider.