Light from the Hidden Sector

Optical precision experiments are a powerful tool to explore hidden sectors of a variety of standard-model extensions with potentially tiny couplings to photons. An important example is given by extensions involving an extra light U(1) gauge degree of freedom, so-called paraphotons, with gauge-kinetic mixing with the normal photon. These models naturally give rise to minicharged particles which can be searched for with optical experiments. In this paper, we study the effects of paraphotons in such experiments. We describe in detail the role of a magnetic field for photon-paraphoton oscillations in models with low-mass minicharged particles. In particular, we find that the upcoming light-shining-through-walls experiments are sensitive to paraphotons and can distinguish them from axion-like particles.