Precision Neutron Decay Measurements

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A free neutron is unstable and decays via the weak interaction into a proton, electron, and antineutrino with a mean lifetime of about 15 minutes. By measuring the energies and directions of the emitted particles and the neutron spin, details of the underlying interaction are revealed. We pass a beam of cold neutrons (moving less than 2000 m/s) through apparatus designed to make precise measurements of the neutron decay lifetime and angular correlations. Beause neutron decay is a relatively simple system, absent the complications of nuclear wave functions, these observables are related to fundamental theoretical parameters directly and unambiguously. We use these results to test predictions of the Standard Electroweak Model and search for evidence of new physics such as scalar/tensor forces or new CP-violation. The precise value of the neutron lifetime is needed to calculate the primordial abundances of light elements produced in the early universe from Big Bang nucleosynthesis.The neutron is a key building block of ordinary matter. When freed from the confines of an atom, the neutron is unstable and will decay into other elementary particles: a proton, an electron, and a lightweight neutral particle called an antineutrino. Neutron decay is a useful laboratory for studying details of the force responsible - the weak nuclear force - one of the four fundamental forces of nature. We pass a beam of free neutrons through specialized detectors that precisely measure the neutron decay rate and the angles between the emitted particles. From this we can test and refine our theory of the weak nuclear force and better understand the physics of the sun, stars, the Big Bang, and important nuclear reactions.