Illuminating the hidden sector of string theory by shining light through a magnetic field

Many models of physics beyond the Standard Model predict minicharged particles to which current and near future low-energy experiments are highly sensitive. Such minicharges arise generically from kinetic mixing in theories containing at least two U(1)U(1) gauge factors. Here, we point out that the required multiple U(1)U(1) factors, the size of kinetic mixing, and suitable matter representations to allow for a detection in the near future occur naturally in the context of string theory embeddings of the Standard Model. A detection of minicharged particles in a low energy experiment would likely be a signal of an underlying string theory and may provide a means of testing it.