Abstract:

We are already used to the idea that significant fractions of the mass in the universe may consist of particles which barely interact with regular, Standard Model matter. The primordial neutrinos, SUSY Wimps, and axions provide ready examples. So the idea that nearly invisible sectors exist is not really foreign. A natural extension is the idea of Hidden Sectors, forces and particles that don't couple directly to the Standard Model. In fact, the dark matter may be part of such a Hidden Sector. The heavy photon, aka dark photon or hidden sector photon, is a force particle in such a hidden sector, a U(1) gauge boson that couples to a "dark" charge, an analogue of electric charge. Dark matter would carry this "dark" charge. The Heavy Photon could be massless or very light, or could have a mass in the 100 MeV range. By virtue of "kinetic mixing", the heavy photon mixes with the Standard Model photon, and so couples, albeit weakly, to electric charge. Consequently, heavy photons will be radiated by electrons, and can decay to electron-positron pairs, but at rates well below those expected for standard QED processes. The Heavy Photon Search (HPS) is an experiment to search for heavy photons in the mass range 10-1000 GeV in fixed target electro-production at Jefferson Lab. We hunt for heavy photons as bumps in e+e- invariant mass, and as long-lived states that produce secondary decay vertices. This talk will outline the motivations for hidden sector photons, review their current experimental status, and discuss in some detail the HPS experiment which is being mounted to look for them.