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) gauge factors. Here, we point out that the required multiple 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.