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About the HPS Experiment

The Heavy Photon Search Group at SLAC is collaborating with physicists at Jefferson Lab, INFN, IN2P3, UNH and UCSC in two experiments aimed at discovering a hidden-sector, heavy photon. Such a particle would have mass in the range 0.01 to 1.0 GeV, couple weakly to electrons, and decay to e ⁺ e ^- . It would be produced by electron bremstrahlung on a heavy target, and be identified as a narrow e ⁺ e ^- resonance. Weak couplings of this heavy photon to electrons account for it having not yet been discovered and can give rise to separated vertices in its decay, providing a spectacular signature. Heavy photons have become a hot topic recently because they may explain high energy electrons and positrons in cosmic rays, and be intimately linked to dark matter annihilation or decay. If the dark matter is part of a hidden sector, as new theories are suggesting,  heavy photons may mediate the  interactions between that hidden sector and our world as well as be responsible for dark matter annihilation and decay.

The first experiment is the APEX experiment, which has been conditionally approved at Jlab, and which has already completed a test run in 2010. The experiment makes use of two large spectrometers in Jlab's experimental Hall A to search for a heavy photon. After successfully producing results, several more data taking runs may be scheduled in the future.

Our focus now is on the second experiment, the Heavy Photon Search (HPS), which was installed at JLAB in February 2015. An engineering run during March and April 2015 will produce our first results. Additional running is planned later in 2015 and 2016 to expand the search for heavy photons over a wide range of possible couplings and masses. The experiment measures forward going electrons and positrons produced in a thin tungsten target with a very high rate silicon tracker/vertexer situated in a dipole magnet. Heavy photons are 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. A lead tungstate crystal calorimeter, situated behind the tracker, provides the fast trigger. The experiment employs the latest in high speed electronics and data acquisiton, and explores new experimental territory, only millimeters away from the incident electron beam.