The DarkLight Experiment: A Precision Search for New Physics at Low Energies

TL;DR

This paper outlines the motivation for a dark photon search at low energies, presents the DarkLight experimental concept for precision $ep$ scattering with full final-state reconstruction, and details a staged path to realization (Phase-I and Phase-II) using existing infrastructure and new detector technologies. It describes leveraging a 100 MeV energy-recovery linac beam on a windowless H$_2$ target inside a 0.5 T solenoidal magnet to achieve high luminosity and ~1 MeV/$c^2$ $e^+e^-$ invariant-mass resolution, enabling sensitivity to $A'$ decays to $e^+e^-$ and invisible final states. The paper covers the motivation, conceptual design, current status, and a phased plan for realization, including detector technologies and a triggerless data-acquisition approach, arguing that the approach can explore previously unconstrained regions of $\\epsilon^2$ vs $m_{A'}$ space. Overall, DarkLight seeks to advance low-$Q^2$ electron scattering and provide a complementary, high-sensitivity search for new light gauge bosons.

Abstract

We describe the current status of the DarkLight experiment at Jefferson Laboratory. DarkLight is motivated by the possibility that a dark photon in the mass range 10 to 100 MeV/c$^2$ could couple the dark sector to the Standard Model. DarkLight will precisely measure electron proton scattering using the 100 MeV electron beam of intensity 5 mA at the Jefferson Laboratory energy recovering linac incident on a windowless gas target of molecular hydrogen. The complete final state including scattered electron, recoil proton, and e+e- pair will be detected. A phase-I experiment has been funded and is expected to take data in the next eighteen months. The complete phase-II experiment is under final design and could run within two years after phase-I is completed. The DarkLight experiment drives development of new technology for beam, target, and detector and provides a new means to carry out electron scattering experiments at low momentum transfers.

Figures (2)

• Figure 1: The 5$\sigma$ limit reached by the complete DarkLight experiment in a run of 1 ab$^{-1}$ integrated luminosity Thaler Searches with negative results at the level of 2$\sigma$ cover the coupling-mass space proposed for DarkLight. DarkLight is unique in searching for $A^\prime$ production from the proton at low masses and will search for invisible decays, which are weakly excluded.
• Figure 2: Schematic layout of the DarkLight experiment with human figure for scale.