According to the standard model of cosmology, Dark Matter constitutes about 84.5% of the total matter in the universe. Unlike normal matter, dark matter does not interact with the electromagnetic force and its existence is only inferred from its gravitational effects on visible matter, radiation, and the large-scale structure of the universe. The most common view is that dark matter is not baryonic, but that it is made up of other, more exotic particles created in the big bang, stable enough to still be around today.

The Heavy Photon Search (HPS) is a new experiment at Thomas Jefferson National Accelerator Facility in Newport News, Virginia, searching for a new U(1) vector boson, called A’ or heavy or dark photon. The dark photon, unlike conventional photons, would have mass and would be detectable only indirectly — after the dark photons have decayed into electrons and positrons (the antimatter counterparts of electrons). Yet, like the familiar photon, which carries the electromagnetic force, the dark photon would carry a force — a new fundamental force in addition to the four that we already know about. It would be the first sign of a hidden sector, which could include entire zoos of new particles, including dark matter. 

The heavy photon would have mass in the range 0.01 to 1.0 GeV, could decay into light dark matter particles (see BDX experiment), and, even if electrically uncharged, could couple weakly to electrons through a mechanism called  kinetic mixing. Thus it could be produced by electron bremsstrahlung on a heavy target, and could decay into an electron-positron pair.The very weak coupling of the heavy photon to electrons accounts for it having not yet been discovered and can give rise to separated vertices in its decay, providing a spectacular signature. 

The HPS experiment measures forward going electrons and positrons produced by electron bremsstrahlung in a thin tungsten target by an intense electron beam of energies between 1.1 and 6.6 GeV. The e+ - e- pairs are identified by a lead tungstate crystal calorimeter, providing the fast trigger, and their momenta and decay vertexes are measured with a very high rate silicon tracker/vertexer situated in a dipole magnet. Heavy photons are thus identified as bumps in the invariant mass spectrum of the electron-positron pairs, and by observing that their decay vertex is separated from the target. The experiment employs the latest in high speed electronics and data acquisition, and explores new experimental territory, only millimeters away from the incident electron beam.



M. Bondì, M. De Napoli, E. Leonora, N. Randazzo.



Thomas Jefferson National Accelerator Facility, Virginia, USA.
Stanford Linear Accelerator Center (SLAC), California, USA. 
Institut de Physique Nucléaire (IPN), Orsay, France. 
Perimeter Institute for Theoretical Physics, Waterloo, Ontario, Canada.