Fundamental Symmetries

As part of the New Standard Model Initiative, TUNL researchers are seeking to shed light on fundamental questions such as Why is there now more visible matter than antimatter in the universe? and What modifications to the electroweak interaction are needed as we develop the New Standard Model? To answer such questions, TUNL researchers are seeking to perform an improved measurement of the permanent electric dipole moment (EDM) of the neutron, as well as continuing in a program of studies of weak decays of neutrons and nuclei.

The matter-antimatter asymmetry that arose during the formation of the early universe could be a result of charge-conjugation-parity (CP) violating interactions that occurred during the electroweak symmetry-breaking phase transition. If such electroweak baryogenesis occurred, new CP-violating interactions must exist beyond what is observed in the neutral kaon and B-meson systems. Almost all new extensions to the Standard Model predict significantly larger EDMs, thus making EDM searches powerful probes of new electroweak CP violation.

To date, extremely precise searches for permanent EDMs of the electron, neutron, and atoms have yielded null results as expected from the Standard Model. We are now in a regime, however, where improvements in sensitivity offer significant discovery potential associated with the θ-term of QCD and CP violation on the electroweak scale. The review of neutron science in the US captures the importance of the EDM experiment: The successful completion of an nEDM experiment, the initiative with the highest scientific priority in US neutron science, would represent an impressive scientific and technical achievement for all of nuclear physics, with ramifications well beyond the field.

Engineering model of the neutron EDM experiment. The neutron beam enters from the left and is incident upon two measurement cells positioned between the central high voltage electrode (red, center) and two ground planes. The constant magnetic field is in the horizontal plane and perpendicular to the beam direction. The horizontal section of the apparatus also contains the 1,000~l helium electrical insulation volume and additional magnetic coils as well as cryogenic and superconducting magnetic shielding. The vertical section of the apparatus houses the dilution refrigerator, helium injection volume, as well as the safety vent system. The entire cryovessel is surrounded by four layers of magnetic shielding. For scale, the height of the beamline is approximately 2m.

TUNL nEDM Activities

TUNL scientists play key roles in next generation measurements in fundamental symmetries tests. We lead efforts in the search for a neutron electric dipole moment, as well as measurements of the neutron lifetime and beta-decay asymmetry coefficients. In addition, we are developing an ultracold neutron source for next-generation beta-decay experiments as well as these experiments themselves.