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.

We present a treatment of the next-to-leading-order radiative corrections to unpolarized Møller and Bhabha scattering without resorting to ultrarelativistic approximations. We extend existing softphoton radiative corrections with new hard-photon bremsstrahlung calculations so that the effect of photon emission is taken into account for any photon energy. This formulation is intended for application in the OLYMPUS experiment and the upcoming DarkLight experiment but is applicable to a broad range of experiments at energies where QED is a sufficient description. DOI: 10.1103/PhysRevD.94.033004 I. MOTIVATIONWith the development of new precision physics experiments on the intensity frontier using lepton beams on targets containing atomic electrons, interest has been renewed in Møller and Bhabha scattering as important signal, background, and luminosity-monitoring processes. Two such experiments are the subject of current attention at the MIT Laboratory for Nuclear Science: DarkLight [1] and OLYMPUS [2]. These experiments require calculations of the Møller and Bhabha processes including next-toleading-order radiative effects.The DarkLight experiment aims to search for a massive dark-sector boson by precisely measuring the process e − p → e − pe þ e − . It will use the 100 MeV electron beam at the Jefferson Lab Low Energy Recirculator Facility incident on a gaseous hydrogen target. DarkLight aims to measure all four final-state particles in a fourfold coincidence. At the design luminosity of ∼10 36 cm −2 s −1 and at such low energies, Møller electrons and associated radiated photons induce an enormous background of secondary particles. Careful study is necessary to understand and minimize the backgrounds masking the comparatively rare signal process.The OLYMPUS experiment aims to measure the ratio of positron-proton to electron-proton elastic scattering cross sections in the effort to quantify the contribution of two-photon exchange. OLYMPUS acquired data with 2 GeV alternating electron and positron beams incident on a hydrogen target [3] at the DORIS storage ring at the Deutsches Elektronen-Synchrotron (DESY). Møller/ Bhabha calorimeters placed at the symmetric angle (90°c .m. ¼ 1.29°l ab ) were used as one of the luminosity monitors. Precise luminosity monitoring is important to normalize the separate electron and positron data sets and form the cross section ratio. Since electron-electron and positron-electron scattering are the only processes in the experiment that can be fully described by QED, they are the most suitable choices for normalization. As a result, knowledge of their cross sections including radiative corrections is essential to forming the final result.A Monte Carlo approach has been identified as the preferred method of treating the radiative corrections for both of these experiments. This approach stands in contrast with traditional soft-photon radiative corrections, which are typically included as a multiplicative factor to the Born cross section,with δ ¼ δðΔE; ΩÞ. This traditional method requ...

The addition of a polarized 3 He ion source for use at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory would enable a host of new measurements, particularly in the context of a planned eRHIC. We are developing such a source using metastability exchange optical pumping to polarize helium-3, which will be then transferred into RHIC's Electron Beam Ion Source for ionization. We aim to deliver nuclear polarization of near 70%, and roughly 10 11 doubly-ionized 3 He ++ ions will be created in each 20 µsec pulse. We discuss the design of the source, and the status of its development.

We detail the design and operation of a compact, discharge light polarimeter for metastability exchange optical pumping of 3 He gas near 1 torr under a low magnetic field. The nuclear polarization of 3 He can be discerned from its electron polarization, measured via the circular polarization of 668 nm discharge light from an RF excitation. This apparatus measures the circular polarization of this very dim discharge light using a nematic liquid crystal wave retarder (LCR) and a high-gain, transimpedance amplified Si photodiode. We outline corrections required in such a measurement, and discuss contributions to its systematic error.

Transfer of polarized 3 He gas across spatially varying magnetic fields will facilitate a new source of polarized 3 He ions for particle accelerators. In this context, depolarization of atoms as they pass through regions of significant transverse field gradients is a major concern. To understand these depolarization effects, we have built a system consisting of a Helmholtz coil pair and a solenoid, both with central magnetic fields of order 30 gauss. The atoms are polarized via metastability exchange optical pumping in the Helmholtz coil and are in diffusive contact via a glass tube with a second test cell in the solenoid. We have carried out measurements of the spin relaxation during transfer of polarization in 3 He at 1 torr by diffusion. We explore the use of measurements of the loss of polarization taken in one cell to infer the polarization in the other cell.