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     The symmetries of three discrete transformations P, T and C play an essential role in the understanding of our universe. After parity was found to be violated in 1957 in weak interactions, CP violation was discovered in the neutral kaon decays. In the Standard Model (SM), the CP violation originates from the complex phase in the Cabibbo-Kobayashi-Maskawa (CKM) matrix, which couples the quarks' weak eigenstates and the mass eigenstates, and the θ term in the QCD Lagrangian. CP violation means T is also violated assuming CPT symmetry. The existence of a non-zero neutron EDM is a violation of P and T simultaneously and the search for a nEDM is a search for CP violation and a search for direct T symmetry violation. While the CP violation in the SM suffices to explain what has been observed in the kaon and B meson systems, it is orders of magnitude smaller than that is needed to explain why the universe is made up of mostly matter instead of equal quantities of matter and anti-matter, also known as the Baryon Asymmetry of the Universe (BAU). One of the three necessary conditions for the baryon asymmetry to occur at the early stage of the universe is CP violation proposed by Sakharov in 1967 and there must be New Physics beyond the SM to account for the extra CP violation. Many other theories, including the supersymmetric (SUSY) extensions of the SM, left-right symmetric models and a class of non-minimal models in the Higgs sector allow for the CP violation mechanisms not in the Standard Model and have their own predictions of the nEDM, which lie between the current experimental upper limit (~2.9 × 10^-26 e⋅cm) and the SM calculations (~2.9 × 10^-31 e⋅cm). New measurements with improved sensitivity of nEDM will help narrow the possible theories of New Physics, have the potential to identify new sources of CP violation and Physics beyond the SM, help to explain the BAU, and also make critical tests of the validity of the SM.

     A new experiment aiming at two orders of magnitude improvement over the current nEDM experimental upper limit has been proposed to be carried out at the Spallation Neutron Source at the Oak Ridge National Laboratory. The experiment will look for a change in the precession frequency of the neutron when placed in external parallel magnetic and electric fields versus that when the electric field direction is reversed. The measurement cell will be made of dTPB-dPS (a wavelength shifting material) coated acrylic and filled with superfluid ⁴ He at ~300-500 mK. Ultra cold neutrons will be produced inside the cell via the super-thermal method. The neutron frequency measurement is carried out by monitoring the scintillation light from polarized ³He-n spin-dependent nuclear process: n+³He → p+t+764 keV. Polarized ³He in the measurement cell will also be used as a co-magnetometer to monitor the in situ magnetic field during the experiment, which is a technique no previous nEDM experiments have used. Both processes require that the ³He polarization have a sufficiently long relaxation time so that polarization loss is negligible during the measurement period in order to achieve the proposed sensitivity. Therefore understanding the relaxation mechanism of polarized ³He in the measurement cell under the nEDM experimental conditions and suppressing the depolarization effect is crucial to the experiment.

      I joined the nEDM collaboration in 2003 and started working on the relaxation time measurement of polarized ³He under the nEDM experimental conditions. The first step to approach the nEDM experimental conditions was to go below the ⁴He lambda point with a dTPB-dPS coated cylindrical cell. Polarized ³He produced via the spin exchange optical pumping (SEOP) method was introduced into a cylindrical dTPB-dPS coated acrylic cell filled with superfluid ⁴He at different levels. Nuclear magnetic resonance (NMR) - adiabatic fast passage (AFP) technique was used to record the time dependence of the ³He polarization to extract the relaxation time. The depolarization probability for polarized ³He at 1.9 K was determined to be ~1.6 × 10^-7 by fitting the experimental relaxation time data to simulations using finite element analysis software. Previous ³He relaxation time measurements by other authors were carried out under various surface conditions at different temperatures. Those measurements seemed to suggest that the depolarization probability had a strong dependence on temperature and in some cases on magnetic field also. The second step was to carry out similar ³He relaxation time measurements at much lower temperatures. A completely redesigned system was built using a dilution refrigerator to cool a rectangular acrylic cell coated with dTPB-dPS to below 400 mK. The rectangular shape is the geometry for the nEDM experiment and the coating technique is completely different from that in cylindrical geometry. Different NMR techniques (AFP and Free Induction Decay) were employed and the extracted depolarization probability was determined to be ~1 × 10^-7, close to the result obtained at 1.9 K. This was the first measurement carried out at this low temperature using an acrylic cell coated with a special wavelength shifting material. The results demonstrated that the proposed nEDM experimental technique using polarized ³He will work, which is a very important step in the funding application from the Department Of Energy for the nEDM experiment. The study of ³He relaxation at 1.9 K and 330 mK involves a wide range of physics knowledge, including the atomic/optical physics to polarize the ³He, magnetic resonance technique to measure the ³He polarization, cryogenic physics to lower the temperature of the experimental cell, surface physics to quantify the coating quality, etc.

     In the nEDM experiment, the ³He density will be ρ=0.8×10¹²/cm³ with a polarization of nearly 100%, a goal that conventional SEOP method cannot achieve. A method which can achieve a polarization of ~100% is to pass an atomic beam of ³He through a region with magnetic field gradients at low temperatures, known as the atomic beam source (ABS). Even though polarized ³He production rate from ABS is orders of magnitude smaller compared to the optical pumping method, it is sufficient for the nEDM experiment. Before polarized ³He atoms from ABS are introduced into the measurement cell, they need to be collected in a reservoir for a short period of time then transferred to the measurement cell. I am currently leading the "Injection Test", whose purpose is to demonstrate that polarized ³He atoms can be injected from the ABS and collected in the collection volume with negligible polarization loss. This is one of the most challenging nEDM R&D experiments in that the pulsed NMR system's sensitivity will be pushed to an extreme and a superfluid ⁴He film burner with a complex geometry will be tested for the first time to suppress the film flow, which is an essential part for the cooling of the whole system.

     In the injection test, polarized ³He will be extremely diluted (~10¹⁴/cm³) in superfluid ⁴He. The ³He NMR signal using AFP method will be too small to detect. The superconducting quantum interference device (SQUID) system is much more sensitive than conventional NMR but it requires complicated magnetic and RF shielding, which is very difficult to incorporate into the injection test apparatus. Pulsed NMR has the sensitivity between the NMR-AFP and the SQUID, and it will be used to measure the ³He polarization. I am currently working on optimizing the signal to noise ratio of the pulsed NMR system in order to measure the low concentration ³He free induction decay signal.

     Suppressing the ⁴He film flow is another challenge in the injection test. Superfluid ⁴He film tends to climb up from the cold region to the warmer part and bring in extra heat load to the part that needs to be cooled down due to the superfluid ⁴He's good thermal conductivity. It is also necessary to keep a high vacuum in the injection tube otherwise ³He atoms coming from the ABS will be deviated from their ballistic trajectories by collisions with ⁴He atoms and result in polarization loss. One way to prevent the ⁴He film from flowing to warmer regions and evaporating is to use a film burner. The working principle is that the helium film is flowing from the low temperature region (~300 mK) to an evaporation plate. A condensing plate will be very close to the evaporation plate. When the film burner is working, the evaporation plate is heated up to maintain a temperature of ~420 mK and the condensing plate is kept below 310 mK. The ⁴He atoms evaporated from the evaporation plate will have a probability of ~100% to strike the condensing plate, stick to it and condense back into superfluid ⁴He. ⁴He atoms will be going through this cycle when the whole system is operating normally so that the ⁴He film flow is stopped from flowing over the film burner to warmer regions. The film burner will generate some heat and the dilution refrigerator needs to have enough cooling power to maintain the temperatures of the evaporation and condensing plates as well as the condensation energy of the helium film to keep the whole system in a dynamically stable state. The preparation of the injection test is ~90% complete and will be carried out in Los Alamos National Laboratory soon.

     One of the systematic uncertainties identified in the nEDM experiment comes from the interaction of the v×E field with the external magnetic field gradients (also called the "Geometric Phase Effect". This interaction produces a frequency shift proportional to the external electric field, mimicking an EDM signal. A general analytical approach based on the relationship between the systematic frequency shift and the velocity autocorrelation function of the stored particles has been developed to describe the effect observed in a recent nEDM experiment at Institute Laue-Langevin. Since the geometric phase effect is highly dependent on the operating conditions of the experiment, a method was proposed by Barabanov, Golub and Lamoreaux to directly measure the correlation function that determines the frequency shift under the exact conditions of a given experiment. The correlation function is directly related to the relaxation rate of polarized ³He at a certain frequency and I have successfully carried out measurements to determine the correlation function at room temperature with a relatively large holding magnetic field. Currently I am working on measuring the correlation function under nEDM experimental conditions using a dilution refrigerator, smaller holding fields and low polarized ³He concentration in a rectangular acrylic cell. This study is very important to the nEDM experiment.

      Understanding the nucleon structure is one of the fundamental tasks in nuclear physics. Low energy Compton scattering experiments can help probe the nucleon polarizabilities, a set of fundamental quantities describing the response of the nucleon to external electromagnetic fields. The High Intensity Gamma Source at Duke Free Electron Laser Laboratory allows us to study these quantities from both polarized and unpolarized nucleon and nuclear targets using polarized photon beams. In the general Compton scattering amplitude, the electric (α) and magnetic (β) polarizabilities enter in terms that are second order in photon energy. The spin polarizabilities, γ₁ to γ₄, enter in third order terms. Proton's α and β have been extracted from low energy Compton scattering measurements on liquid hydrogen targets. However due to the lack of free neutron targets in nature, deuterium targets have been used to extract neutron's electric and magnetic polarizabilities. The knowledge of nucleon's spin polarizabilities are much poorer than α and β. Only γ₀=γ₁-γ₂-2γ₄ and γ_π=γ₁+γ₂+2γ₄ have been extracted so far and more measurements sensitive to nucleon's spin polarizabilities are needed. A polarized ³He nucleus' ground state is dominated by the S wave in which the spin of the ³He nucleus is carried by the unpaired neutron and this makes polarized ³He an effective polarized neutron target. Using a polarized ³He target and the polarized photon beam from HIγS facility at DFELL, the spin polarizabilities of the neutron can be extracted for the first time.

     There is an important Gerasimov-Drell-Hearn (GDH) sum rule which connects the helicity difference in the total photoabsorption cross section of circularly polarized photons on a longitudinally polarized nucleus with the anomalous magnetic moment of the nucleus. The GDH integral on ³He from the two-body production threshold (~5.5 MeV) to the pion production threshold (~140 MeV) is particularly interesting due to a big discrepancy between the state-of-the-art three-body calculations' predicted value in this part of the integral and the estimated value from the experimental data. A polarized ³He target can be used at HIγS for stringent tests of these three-body calculations including effective field theory calculations in the future.

     I am mainly involved in constructing, testing and improving the polarized ³He target used at HIγS. In 2008, the first measurement of double polarized three-body photodisintegration of ³He was carried out with 11.4 MeV photon beam. In this experiment, a high-pressure and longitudinally polarized ³He gas target and a circularly polarized photon beam were employed. Seven liquid scintillating neutron detectors were placed around the ³He target to detect the neutrons from the three-body breakup channel. The data is being analyzed and the results will be published soon. The ³He gas target cell was made of aluminosilicate (GE180) glass. This type of glass has fewer magnetic impurities and is less permeable to ³He atoms than regular pyrex glass. However, the rich concentration of barium in the GE180 glass produced a large amount of background events in the neutron detectors. To reduce the background for future measurements at HIγS, a new high-pressure ³He cell made of sol-gel coated pyrex glass has been developed and tested. Using NMR-AFP and electron paramagnetic resonance (EPR) techniques, the highest measured polarization of the cell has been measured to be ~67%, which is in good agreement with theoretical predictions. This is the first time that sol-gel coating technique has been applied to a high-pressure ³He target cell for nuclear physics experiments. In May 2009, an in-beam background test showed that the sol-gel coated pyrex glass target generated fewer neutron background events than the GE180 glass cell. This new type of target will be of great help for the Compton scattering and GDH experiments at HIγS.