APEX: A Prime EXperiment at Jefferson Lab

APEX is an experiment at Thomas Jefferson National Accelerator Facility (JLab) in Virginia, USA, that searches for a new gauge boson ($A^\prime$) with sub-GeV mass and coupling to ordinary matter of $g^\prime \sim (10^{-6} - 10^{-2}) e$. Electrons impinge upon a fixed target of high-Z material. An $A^\prime$ is produced via a process analogous to photon bremsstrahlung, decaying to an $e^+ e^-$ pair. A test run was held in July of 2010, covering $m_{A^\prime}$ = 175 to 250 MeV and couplings $g^\prime/e \; \textgreater \; 10^{-3}$. A full run is approved and will cover $m_{A^\prime} \sim$ 65 to 525 MeV and $g^\prime/e \; \textgreater \; 2.3 \times10^{-4}$.