High precision cold neutron beta decay correlation measurements at the SNS; the Nab experiment.

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A major focus of the current research effort within the subatomic physics community is the verification of and search for possible extensions to the so called Standard Model of particle physics, which seeks to predict and describe the fundamental building blocks of matter and their interactions. Our current understanding of all physical phenomena is based on four fundamental interactions: gravitation, the electromagnetic interaction, the strong interaction, and the weak interaction. The weak interaction takes a special role among the four known forces, as it is the only one known to violate parity. Generally, parity is a fundamental symmetry in nature, relating processes and their mirror images. The work proposed here involves studies which use the weak interaction between subatomic particles as a probe for their fundamental properties. The experiments use high intensity neutron sources and particle accelerators to either search for very small spatial and yield asymmetries in the products of the reaction between the projectile and a given target, or to measure with high accuracy the properties of particle (i.e. neutron) decay products. The fact that only the weak interaction violates parity, and its short range, makes these measurements highly useful probes of very small effects inside nucleons. The currently accepted mathematical and phenomenological description of our knowledge about the interactions between subatomic particles is known as the Standard Model. Although highly successful, there are strong theoretical reasons, such as the large number of adjustable parameters, to expect that the Standard Model is a low energy effective theory of some more fundamental description of nature. The proposed project will consist of measurements using cold neutrons, conducted at the Spallation Neutron Source in Oak Ridge, Tennessee. This world-leading new facility will permit us to carry out a program of ultra-precise measurements of weak interactions of the neutron.