Heavy Photon Search (HPS)
The astrophysical evidence for Dark Matter is compelling, but so far there is no evidence that it has been produced in particle colliders, interacted with sensitive detectors on Earth, or been seen directly in the cosmos.
The inability of experiments to find dark matter challenges the theory of supersymmetry, which predicts the existence of new, weakly interacting massive particles which could constitute the dark matter. A competing theory proposes that dark matter is part of a so-called “hidden sector” – a part of the universe that doesn't readily interact with our material universe. Hidden sectors could co-exist with our slice of the universe, but would remain undetected because their interactions with regular matter are so weak. If dark matter is part of a hidden sector, it could interact via the “heavy photon,” a hidden sector version of our regular photon. (Because these photons have a tiny bit of mass—as opposed to the regular photon’s none—they’re called “heavy” photons.) The Heavy Photon Search (HPS) hunts for such heavy photons.
HPS will collide high-energy electrons with a thin tungsten target, copiously producing pairs of electrons and positrons among other debris. Because heavy photons could also be produced in this process, and would decay into electron and positron pairs, HPS will collect a huge amount of data and look for an excess of pairs at specific masses and locations that could only have been caused by heavy photon decay.
The HPS experiment employs silicon microstrip detectors placed inside a large magnet to measure the momentum and decay location of electron and positron pairs. SLAC led the development and construction of that detector, called the “Silicon Vertex Tracker.” It is designed to track the electron-positron pairs, but also identify the location where they pop into being, called the vertex. If the vertex appears some distance beyond the target foil, an unseen particle—like a heavy photon—was created first, then flew a certain distance before decaying into the electron–positron pair. The experiment also depends on a highly segmented crystal “Electromagnetic Calorimeter,” built by HPS collaborators from the Thomas Jefferson National Accelerator Facility and European institutions, to measure the energy of the escaping particles and trigger on likely heavy photon decays. The whole experiment takes data at very high rates in order to accumulate the huge data samples needed to search for a rare process in a reasonable amount of time.
The experiment runs at Jefferson Lab, and uses the intense multi-GeV electron beams produced by its CEBAF accelerator. The HPS Collaboration includes physicists from Jefferson Lab, SLAC, the Laboratoire de Physique Théorique d'Orsay, and the Istituto Nazionale di Fisica Nucleare, in addition to many US universities that work at Jefferson Lab.