DarkLight: A Search for Dark Forces at the Jefferson Laboratory Free-Electron Laser Facility

TL;DR

This paper presents the DarkLight detector concept to search for a heavy photon $A^{\prime}$ in the mass window $10~\mathrm{MeV}/c^2 < m_{A^{\prime}} < 90~\mathrm{MeV}/c^2$ and coupling $10^{-9} < \alpha^{\prime} < 10^{-6}$ via $A^{\prime} \to e^+e^-$ decays in $ep$ scattering at the Jefferson Lab Free Electron Laser facility. The design features a compact magnetic spectrometer with a windowless hydrogen target inside a $0.5$ T field and multi-layer tracking plus photon detection to suppress irreducible QED backgrounds through precise momentum and energy measurements. It also discusses extending sensitivity to invisible decays with a Pb-scintillator detector to probe the $(g-2)_\mu$-preferred region, including feasibility estimates and limitations from photons escaping along the beamline. The work reports FEL beam-test results validating high-power transmission and outlines a development path through PAC approvals toward commissioning around 2015–2016, highlighting a complementary probe of dark forces in an under-explored parameter space.

Abstract

We give a short overview of the DarkLight detector concept which is designed to search for a heavy photon A' with a mass in the range 10 MeV/c^2 < m(A') < 90 MeV/c^2 and which decays to lepton pairs. We describe the intended operating environment, the Jefferson Laboratory free electon laser, and a way to extend DarkLight's reach using A' --> invisible decays.

Figures (4)

• Figure 1: Left: A subset of the $A^{\prime}$ parameter space of interest to DarkLight. The shaded area is the sum of regions excluded by existing bounds on the electron magnetic moment $a_e$ and the muon magnetic moment $a_{\mu}$, and by beam dump experiments. The curve shows the expected reach for DarkLight. The dotted region is the $a_{\mu}$ preferred region. Observation of an $A^{\prime}$ in this region could explain the known $(g-2)_{\mu}$ anomaly. Right: Schematic cross-sectional view of the DarkLight detector identifying its main components.
• Figure 2: Layout of the JLab FEL. DarkLight will be located at position marked A1 on the UV beamline.
• Figure 3: DarkLight invisible search reach for various photon efficiencies. The gray shaded area indicates constraints from anomalous magnetic moment measurements, with the green region indicating the "welcome" region where an $A'$ could explain the $(g-2)_\mu$ discrepancy. $\Delta_{\rm{cut}} = 1$ refers to a kinematic cut designed to mitigate mis-measurement of the reconstructed $A'$ invariant mass. The beam dump constraints from the visible search are absent here because they do not apply to invisibly decaying particles.
• Figure 4: Simulated detection capabilities of 3-layer Pb-scintillator detector concept.