The UCN facility’s first experiment is searching for the neutron electric dipole moment (nEDM).
To account for the observed matter-antimatter asymmetry of the universe, new sources of CP violation beyond the Standard Model are required. The small amount of CP violation in the Standard Model leads to a very tiny nEDM of about 10−31 e-cm. However, in several models of physics beyond the Standard Model, extra sources of CP violation are present, and notably, these models often generate nEDM’s at the 10−27 e-cm level.
The current experimental limit of detection for the nEDM is about 3×10−26 e-cm. TRIUMF’s nEDM experiment, and other next-generation neutron experiments around the world, aim to constrain the nEDM to roughly the 10−27 e-cm level. Critically, this approximately 300 times greater level of nEDM sensitivity will confirm, or reject, a variety of beyond-Standard Model theories.
UCN measurements of the nEDM use the fact that the neutron has a magnetic dipole moment like a bar magnet. When placed in a magnetic field, the neutron’s spin axis precesses about the direction of the magnetic field the same way that a spinning top precesses about the direction of gravity.
If an electric field is applied in the same direction as the magnetic field, then, if the neutron has a non-zero EDM, it will change the precession frequency. By using a technique called Ramsey Resonance, the UCN experiment will make very precise measures of the precession frequency, and any change when the electric field is reversed.
The Neutron Lifetime Experiment
A second key experiment envisioned at TRIUMF’s UCN facility is ultra-precise measurement of the neutron lifetime. Free neutrons decay with a half-life of about 15 minutes into a proton, an electron, and an electron anti-neutrino. However, at present, there’s a significant discrepancy between the neutron half-life from beam measurements using cold neutrons and bottle experiments using ultracold neutrons. Beam experiments tend to predict a longer neutron lifetime than storage experiments. As a result, the world average neutron half-life calculated by the Particle Data Group has an error inflated by a factor 1.9. (For a detailed listing of neutron measurements see here).
A more accurate and precise determination of the neutron lifetime is important for two key reasons:
- The neutron lifetime is an essential parameter for Big Bang nucleosynthesis calculations and is currently the major uncertainty for accurate predictions.
- The rate of neutron decay is strongly correlated to the intensity of the weak interaction. At present, the weak force is the least precisely measured of the fundamental constant forces (weak, strong, electromagnetic and gravity). The ability to confine ultracold neutrons will open the possibility for measuring neutron half-lives with much greater accuracy, thus providing an improved understanding of the strength of the weak interaction.
TRIUMF’s TUCAN (TRIUMF ultracold advanced neutron source) collaboration includes the University of British Columbia, Simon Fraser University University of Winnipeg, University of Manitoba, University of Northern British Columbia, Nagoya University in Japan and Japanese Laboratories, KEK, the High Energy Accelerator Research Organization, RCNP, the Research Center for Nuclear Physics, and the University of Osaka.
Why ultracold neutrons?
Ultracold neutrons (UCN) are required for TRIUMF’s neutron experiments because these neutrons are moving so slowly they can be contained in a bottle, and thus precisely measured.
For a free, fast neutron at room temperature, what to us looks like a solid wall of metal is an open door. For example, if fast neutrons hit a 4-mm thick nickel plate, about 9-in-10 will go straight through without any interaction. This because the solid-looking nickel is actually tiny nickel nuclei separated by vast empty space. The inter-atomic spacing in solid nickel is 10,000 femtometers whereas the nickel nuclei are only about 10 femtometers in diameter. Thus, a neutron has to make a direct hit on a nucleus to be slowed, and this is a rare event.
However, as the neutron becomes colder, and its speed reduces, it becomes more and more likely to interact with other matter. Quantum mechanically, a neutron exists as both a wave and a particle. Ultracold neutrons have such a long wavelength that they interact with hundreds of nuclei at a time. In the TRIUMF UCN experiments, they are so cold and moving so slowly that they bounce off solid materials. This enables TRIUMF scientists to contain the ultracold neutrons in the same way a gas is held in a metal bottle.