Precision Measurementswith Low Energy Neutrons

This proposal is a renewal of NSF-0855584, “Fundamental symmetry tests using low energy neutrons,” funded $300,000 from 2009-08-01 to 2012-07-31. It was for two projects: a) nEDM – a search for the electric dipole moment dn of the neutron, and b) NPDGamma – measurement of the longrange weak pion coupling constant fð. It supported a graduate student working on NPDGamma (3 years) and nEDM (2 years), two undergraduate summer students, the PI (summer), and travel. During this period, our group developed new techniques for the design and construction of guide field magnets for the nEDM experiment. Initial success of our design of a neutron guide field led to extensions of our method to design guide fields for 3He injection from the atomic beam source (ABS), and for 3He transfer from the collection volume to the measurement cell. Our group played a leading role in construction and commissioning of the NPDGamma experimental apparatus at the Fundamental Neutron Physics Beamline (FnPB) at the SNS. All components except for the cryogenic hydrogen target have been installed and commissioned. We have measured the neutron intensity and polarization profiles, and measured parity-violating (PV) spin asymmetries from neutron capture on 35Cl and 27Al in preparation for the measurement of the hydrogen capture asymmetry Aã. See sections 2.2 and 3.1 for a detailed discussion of our results. We seek additional funding during the next three years to a) complete data-taking of the NPDGamma experiment and publish our measurement of fð; b) construct and test a data acquisition (DAQ) system for the Nab experiment – a measurement of the unpolarized decay correlations a and b of the neutron; and c) construct and test prototype guide field coils for the nEDM experiment. Each experiment is at a different stage in development, and all three have been endorsed by Nuclear Science Advisory Committee (NSAC) and approved by the FnPB Proposal Review and Advisory Committee (PRAC) at Oak Ridge National Laboratory (ORNL). The common physics in these experiments is precision measurements of discrete symmetries in neutron systems. Symmetries play a vital role in nuclear and particle physics. Interactions are characterized by their underlying symmetries, including both continuous symmetries of space-time and the accompanying fields [1, 2], as well as the discrete symmetries parity (P), time reversal (T), charge conjugation (C), and particle exchange. Profound discoveries have come through investigation of these symmetries, for example special relativity, generalized angular momentum, antiparticles, and the P- and CP-violating nature of the weak interaction. Effective field theories are based on the spontaneous breaking of chiral symmetry in QCD, while the mass of particles is generated by spontaneous symmetry breaking. Low-energy neutrons are ideal for precision measurements of symmetries. They are insensitive to Coulomb effects, but can be manipulated magnetically. They interact both strongly and weakly. The neutron and proton form an isospin doublet. The slightly heavier free neutron decays with a lifetime long enough to perform precision experiments on either in-beam cold neutrons or trapped ultra cold neutrons (UCN). The proposed symmetry measurements on the neutron investigate both hadronic structure within the Standard Model (SM), and are sensitive to physics beyond the SM. A common aspect of all precision neutron experiments is the careful generation and control of static and RF magnetic fields. The technique we developed for designing precision magnets is based on a new physical interpretation of the magnetic scalar potential. It is general, not limited to nuclear physics. This proposal includes activities to refine and extend the scope of this technique. We will apply it to the design and construction of precision coils for the EDM experiment as well as the R&D of novel spin manipulators for other parity polarized neutron experiments. This includes development of technology to construct coils in the form of 3-d printed circuits using computer numerical control (CNC).