Are there ultralight particles in the hidden sector?
Most embeddings of the standard model into a more unified theory, in particular the ones based on supergravity or superstrings, predict the existence of a hidden sector of particles and interactions which have only very weak interactions with the visible sector standard model particles. The gauge interactions in the hidden sector generically involve several U(1) factors. Usually, it is assumed that the corresponding gauge bosons are very heavy, in order to avoid eventual observational constraints from low energy experiments. However, in realistic string compactifications, some of these hidden photons may indeed be light, with masses in the sub-eV range. In this case, the dominant interaction with the visible sector photon will be through gauge kinetic mixing. Moreover, hidden sector matter particles charged under such U(1)s will acquire some effective electric charge proportional to the kinetic mixing angle. Correspondingly, such light hidden sector particles may be searched for in experiments exploiting high fluxes of low energy photons and/or large electromagnetic fields. We will present a review of this emerging low energy frontier of fundamental physics.