C © Owned by the authors, published by EDP Sciences, 2014 APEX: A Prime EXperiment at Jefferson Lab

Abstract. APEX is an experiment at Thomas Jefferson National Accelerator Facility (JLab) in Virginia, USA, that searches for a new gauge boson (A′) with sub-GeV mass and coupling to ordinary matter of g ′ ∼ (10−6 − 10−2)e. Electrons impinge upon a fixed target of high-Z material. An A ′ 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 mA ′ = 175 to 250 MeV and couplings g′/e> 10−3. A full run is approved and will cover mA ′ ∼65 to 525 MeV and g′/e> 2.3 × 10−4. 1. Motivations The Standard Model (SM) of particle interactions is described by an SU(3)C × SU(2)L × U(1)Y gauge group, where the forces are mediated by vector bosons. For an extension of this model to have thus far evaded detection the corresponding gauge boson must have a mass of O(TeV) or must be very weakly coupled to ordinary matter, with a coupling strength g ′ suppressed relative to the electromagnetic charge e by ≡ g′/e ∼ 10−6 − 10−2 [1] (or, equivalently, ′/ = 2). This new gauge boson, A′, corresponding to a U(1) ′ extension of the SM can acquire an effective interaction with electromagnetism via kinetic mixing, where quantum loops of arbitrarily heavy particles provide a means by which th