Abstract
Jefferson Lab’s A ′ Experiment (APEX) will search for a new vector boson, the A ′, in the mass range 65 MeV A ′ ′ coupling to electrons of α ′ > 6 × 10−8 α , where α = e 2 /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 + 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.
Highlights
The experiment will search a kinematic regime in which the A can decay to an electronpositron pair; electron-positron pairs will be detected in the twin High Resolution Spectrometers positioned at small angles relative to the Hall A incident electron beam
APEX is designed to reach the required invariant mass resolution, 0.5%, for forward-peaked lepton pairs by using three key ingredients: 1) JLab’s twin high-resolution spectrometers (HRS left and High Resolution Spectrometers (HRSs) right) with improved rate capability trackers, 2) a septum magnet, shown schematically in Fig. 1, that will bend the electron-positron pairs produced around ±7.5◦ outwards and into the acceptance of the spectrometers, and 3) a novel multi-foil tungsten target
The analysis magnetic-optics parameters were tuned to accurately reconstruct the hole pattern using the trajectories of the electrons that had passed through the holes
Summary
The experiment will search a kinematic regime in which the A can decay to an electronpositron pair; electron-positron pairs will be detected in the twin High Resolution Spectrometers positioned at small angles relative to the Hall A incident electron beam. APEX is designed to reach the required invariant mass resolution, 0.5%, for forward-peaked lepton pairs by using three key ingredients: 1) JLab’s twin high-resolution spectrometers (HRS left and HRS right) with improved rate capability trackers, 2) a septum magnet, shown schematically, that will bend the electron-positron pairs produced around ±7.5◦ outwards and into the acceptance of the spectrometers, and 3) a novel multi-foil tungsten target APEX is designed to reach the required invariant mass resolution, 0.5%, for forward-peaked lepton pairs by using three key ingredients: 1) JLab’s twin high-resolution spectrometers (HRS left and HRS right) with improved rate capability trackers, 2) a septum magnet, shown schematically in Fig. 1, that will bend the electron-positron pairs produced around ±7.5◦ outwards and into the acceptance of the spectrometers, and 3) a novel multi-foil tungsten target
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