Theory of the Hidden Photons



A Hidden Photon (HP) is the gauge boson of a local U(1) hidden symmetry. Such symmetries arise in popular extensions of the Standard Model, especially in String Theory. Known particles have no direct interaction with HPs (hence 'hidden'), still couplings can be generated through radiative corrections or gravity. In particular HPs may not be fully hidden, as very massive particles with both electric and hidden charge can generate kinetic mixing with the standard photon. In this case, the natural value of the dimensionless kinetic mixing coupling is that of a quantum correction. Since the hidden gauge coupling can be very small, and because of possible cancellations between different mediator contributions, there is no clear minimum for the coupling. Values in the 10-16 - 10-3 range have been predicted in the literature. Moreover, HPs can become massive via the Higgs and/or Stueckelberg mechanisms. The very weak interaction makes HPs perfect candidates for the dark sector that current cosmology and astrophysics are revealing. They have been proposed as Dark Matter (DM) candidates and as mediating Dark Forces between DM particles. On the other hand, if their mass is in the meV range, their cosmological relic abundance could also provide the extra radiation favoured by the recent WMAP-7 results. The presence of the kinetic mixing term signals that photon and HP fields are not orthogonal. Since the photon is by definition an interaction eigenstate (couples to the electric charge) and the HP is in general massive, the kinetic mixing misaligns the interaction and propagation eigenstates. Most importantly, the states orthogonal to propagation eigenstates are completely sterile to electromagnetic interactions. The misalignment between interaction and propagation eigenstates is known to produce flavour oscillations, i.e. photons will convert into sterile states. This oscillation mechanism enables us to search for subeV mass HPs.





© 2014, by Javier Redondo &MS, GW, Hamburger Sternwarte