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.

A test of parity-conserving, time-reversal noninvariance ͑PC TRNI͒ has been performed in 5.9 MeV polarized neutron transmission through nuclear spin aligned holmium. The experiment searches for the T-violating fivefold correlation via a double modulation technique-flipping the neutron spin while rotating the alignment axis of the holmium. Relative cross sections for spin-up and spin-down neutrons are found to be equal to within 1.2ϫ10 Ϫ5 ͑80% confidence͒. This is a two orders of magnitude improvement compared to traditional detailed balance studies of time reversal, and represents the most precise test of PC TRNI in a dynamical process, to our knowledge. ͓S0556-2813͑97͒06905-7͔

Dense samples (10–100barcm) of nuclear spin polarized He3 are utilized in high energy physics, neutron scattering, atomic physics, and magnetic resonance imaging. Metastability exchange optical pumping can rapidly produce high He3 polarizations (≈80%) at low pressures (few mbar). We describe a polarized He3 gas compressor system which accepts 0.26barlh−1 of He3 gas polarized to 70% by a 4W neodymium doped lanthanum magnesium hexaluminate (Nd:LMA) laser and compresses it into a 5barcm target with final polarization of 55%. The spin relaxation rates of the system’s components have been measured using nuclear magnetic resonance and a model of the He3 polarization loss based on the measured relaxation rates and the gas flow is in agreement with a He3 polarization measurement using neutron transmission.

Measurements have been made of ACTT for polarized neutrons incident on a polarized proton target from 3.65 to 11.60 MeV. In the energy range near 10 MeV, ACTT is very sensitive to the nucleon-nucleon tensor interaction. Comparison of the data to potential-model predictions indicate that the tensor interaction is weak, resulting in values of the ^Si-^Di mixing parameter €i which are smaller than predicted by any nucleon-nucleon potential model. A smaller tensor force will bring the predictions of local potential models for the triton binding energy into closer agreement with the experimental value.PACS numbers: 25.40.Dn, 13.75.Cs, 21.30.+y, 25.10.+S A long-standing problem in nuclear physics has been the theoretical prediction of the triton binding energy [1]. All realistic and local nucleon-nucleon (NN) potentials predict too little binding. Many mechanisms have been proposed to increase the triton binding energy, among which are three-body forces and relativistic effects. Recent theoretical work, however, suggests that neither mechanism can contribute significantly [2,3]. In contrast, it is well established that the strength of the NN tensor force below 50 MeV has a large influence on the triton binding energy [4]. A^A^ parameters above 50 MeV are not important for this problem as this is approximately the Fermi energy of the nucleons in the triton. Calculations [5] show that a weak tensor force at low energies will bring potential model predictions into closer agreement with the experimentally determined value.In spite of its importance, both in the binding of fewnucleon systems and in the saturation of nuclear matter [5], the strength of the NN tensor force is only loosely constrained by the existing data [6]. We have measured the spin-dependent difference in total cross section, Aor, for the scattering of transversely polarized neutrons from transversely polarized protons below 12 MeV. These measurements cover a significant fraction of the energy range important to the triton binding energy. Our measurements indicate that the tensor force is indeed weak at low energies, possibly resolving the triton binding energy problem.At low energies, the strength of the tensor interaction is parametrized by the isoscalar ^Si-^Di phase-shift mixing parameter ei. Aar exhibits a large sensitivity to ei for neutron energies from 5 to 35 MeV [7]. In addition, ACT is insensitive to most other phase-shift parameters. The only significant sensitivity is to the ^SQ and ^Si phase shifts which are constrained by experiment. In contrast, other observables sensitive to the tensor interaction such as the spin transfer parameter, K^ , and the spin-correlation coefficient, Ayy{9)^ are sensitive to the P waves, which are much less well determined. Only in the special case 9 = 90^ is Ayy independent of the -^Pi phase shift.ACT is measured using a polarized neutron beam incident upon a polarized proton target and is defined to be the total cross section with the spins antiparallel minus the total cross section with the spins parallel [8]:A...