Session EJ: Mini-Symposium on Fundamental Neutron Physics III

Symmetry Violation Searches with Neutrons

Neutrons are a great tool for fundamental property investigations, such as symmetries. Within this context they are a complementary probe to the standard models of particle physics and cosmology as compared to the high energy frontier.

Magnetic field apparatus for the neutron electric dipole moment search at Oak Ridge National Lab

A search for a neutron electric dipole moment (nEDM) will be carried out at the Oak Ridge National Laboratory's Spallation Neutron Source (SNS) with a sensitivity goal of < 3 × 10^-28 e-cm. Polarized ultracold neutrons will precess in a constant magnetic holding field, B₀, inside a ∼0.4 m long liquid helium cell, which will be doped with a minute amount of He-3 to measure the precession frequency. Magnetic field gradients in the cell must be reduced below 3 ppm/cm relative to B₀ in order to mitigate the false nEDM signal due to the geometric phase effect and to increase the neutron and He-3 polarization lifetime. Superconducting magnetic coils ∼1.5-2 m in diameter will be operated inside a nearly-hermetic, superconducting lead shield to produce B₀ and other required fields. I will discuss design efforts to meet the stringent magnetic requirement in ways that also meet the challenges of cryogenic operation, in particular the significant thermal contraction. I will also show ongoing work to prepare and validate the experiment's main aluminum cryostat for use. 

Producing ultracold neutrons with a spallation source and superfluid helium

An electric dipole moment of the neutron large enough to detect with current techniques would be an indication of CP-violating processes beyond the standard model of particle physics and could explain the matter-antimatter asymmetry in the universe.

The goal of the TUCAN collaboration is to measure the neutron electric dipole moment with a sensitivity of 10^-27 ecm. To achieve the necessary statistical sensitivity, we are designing the TRIUMF UltraCold Advanced Neutron (TUCAN) source, a next-generation source for ultracold neutrons based on a spallation target and a superfluid-helium converter. To this end, we recently completed the installation of a prototype source at a new, dedicated beamline at TRIUMF.

This presentation will show results from a comprehensive test run of the prototype source, compare them to simulations, and show how we use the simulations to optimize neutron moderators for the next-generation source to achieve a several orders of magnitude higher ultracold-neutron flux.   

129Xe comagnetometer for neutron EDM experiment

The neutron electric dipole moment (nEDM) has been providing strict constraints on theories underlying the CP violation that predict larger nEDM than the standard model. In order to exceed the present upper limit on the nEDM of 3 × 10⁻²⁶ ecm measured at Grenoble, several groups are now developing new methods of nEDM measurements to reduce systematic errors as well as new ultracold neutron (UCN) sources to achieve higher statistics. The systematic error at the Grenoble experiment was dominated by the the geometric phase effect (GPE) for the ¹⁹⁹Hg comagnetometer. We have proposed a ¹²⁹Xe comagnetometer which can possibly reduce the GPE to 4 × 10⁻²⁸ ecm because of a buffer gas effect. Spin polarized ¹²⁹Xe nuclei are used to monitor the static magnetic field in an nEDM cell where both spin polarized UCN and ¹²⁹Xe are confined. Highly spin polarized ¹²⁹Xe produced by means of the spin exchange optical pumping (SEOP) technique is accumulated in a cold trap, and then is introduced into the nEDM cell. The spin precession of ¹²⁹Xe nuclei will be observed using two-photon excitation of Xe atoms. We have achieved 70% of ¹²⁹Xe polarization in the SEOP cell and are now developing the cold trap. The progress of our development will be presented.