Axion-like particle oscillations.

String theory compactifications may generate a large number of light axion-like particles (ALPs) whose weak coupling to electromagnetism could be experimentally observable. In general, a large number of ALPs may exist, with a linear combination having a potentially observable coupling to electromagnetism. The basis in which only one ALP couples to electromagnetism is in general misaligned with the mass basis. This leads to mixing between the 'electromagnetic' ALP and a number of 'hidden' ALPs that do not interact directly with the photon. The process is analagous to neutrino oscillations. I will discuss the phenomenological consequences of this mixing, in particular showing that it may significantly reduce the predicted signal in experiments such as CAST.