The APEX experiment at JLab. Searching for the vector boson A′ decaying to e⁺e⁻

Dept. of Physics; Carnegie Mellon University ; Pittsburgh, PA 15213, USA

^* e-mail: gbfranklin@cmu.edu

Published online: 12 April 2017

Abstract

Jefferson Lab’s A′ Experiment (APEX) will search for a new vector boson, the A′, in the mass range 65 MeV < m_A′ < 550 MeV, with sensitivity for the A′ coupling to electrons of α′ > 6 × 10⁻⁸α, where α = e²/4π. New vector bosons with such small couplings arise naturally from a small kinetic mixing of the “dark photon”, A′, with the photon — one of the very few ways in which new forces can couple to the Standard Model — and have received considerable attention as an explanation of various dark-matter related anomalies. In this experiment, A′ bosons produced by radiation off an electron beam could appear as narrow resonances with small production cross-sections in the e⁺e⁻ invariant mass distribution. The two Jefferson Lab HRS spectrometers will provide a reconstructed invariant-mass resolution for the A′ of δM/M < 0.5%. With a 33-day run, the experiment will achieve high sensitivity by taking advantage of this mass resolution and high statistics of the e⁺e⁻ pairs, which will be orders of magnitude larger than in previous searches for the A′ boson in this mass range. This paper will review the key concepts of the experiment and the status of the preparations for running APEX. The results of a completed pilot run will be presented.