Neutrons and Fundamental Symmetries

Abstract

The research supported by this project addressed fundamental open physics questions via experiments with subatomic particles. In particular, neutrons constitute an especially ideal “laboratory” for fundamental physics tests, as their sensitivities to the four known forces of nature permit a broad range of tests of the so-called “Standard Model”, our current best physics model for the interactions of subatomic particles. Although the Standard Model has been a triumphant success for physics, it does not provide satisfactory answers to some of the most fundamental open questions in physics, such as: are there additional forces of nature beyond the gravitational, electromagnetic, weak nuclear, and strong nuclear forces?, or why does our universe consist of more matter than anti-matter?Although the neutrons bound within atomic nuclei (such as iron, aluminum, etc.) are stable, neutrons liberated from the nuclei of atoms undergo radioactive decay, with a lifetime of approximately fifteen minutes. Under the Standard Model, the decay of the neutron proceeds via the weak nuclear force, and theoretical predictions for how the decay proceeds can then be compared with experimental measurements. An inconsistency between the Standard Model prediction and experimental results would provide evidence for an additional force mediating the decay of the neutron. Thismore » proposal supported experimental work on measurements of the decay of the neutron to be carried out at the Los Alamos National Laboratory in New Mexico. The results from this experimental work have provided important input to an assessment of the limits of the Standard Model’s validity. Finally, under the Standard Model, for every subatomic particle comprising “ordinary” matter, there is a corresponding “anti-particle” comprising “anti-matter”. For example, the antiparticle of the electron is the positron, the basis of Positron Emission Tomography (PET) scans in medicine. Despite the known existence of these anti-particles, it is not understood how the universe evolved from its beginning at the Big Bang with presumably equal numbers of particles and anti-particles to its present ordinary-matter-dominated state, consisting of significantly more particles than anti-particles. Experiments searching for a so-called “electric dipole moment” of the neutron, resulting from a tiny separation of positive and negative electric charge within the electrically-neutral neutron, are poised to address this question. In particular, the interactions responsible for the existence of such an electric dipole moment are closely related to the physics processes necessary for the generation of more matter than anti-matter during the early evolution of the universe. Thus, the discovery of a non-zero neutron electric dipole moment would provide key insight into the question of why we live in a matter-dominated universe. This proposal supported work towards the development of a new experiment to search for a neutron electric dipole moment at the Spallation Neutron Source at the Oak Ridge National Laboratory in Tennessee. The experiment requires precise knowledge of the magnetic field within the experimental apparatus. One of the primary novel and original results from this project included the development of a new technique which permits a determination of the magnetic field within the apparatus solely from non-invasive measurements of magnetic fields in regions located external to the measurement volume located in the internal region of the apparatus. This project also contributed significantly to the training of the next generation of scientists, of considerable value to the public. Young scientists, ranging from undergraduate students to graduate students to post-doctoral researchers, made significant contributions to the work carried out under this project.« less