Physics - Medium and High Energy Seminar

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Medium and High Energy Physics Seminar: Chen-Yu Liu (University of Indiana) "A Modern Measurement of the Neutron Lifetime Using Ultracold Neutrons in a Magneto-gravitational Trap"

Event Type
464 Loomis
Feb 24, 2020   1:00 pm  
Chen-Yu Liu (University of Indiana)
Brandy Shier

Neutron properties determine important physics on scales ranging from the very small (sub-atomic structure) to the very large (cosmic evolution).  The neutron decay rate set the time scale for Big Bang Nucleosynthesis and determined the cosmic abundance of light elements.  High precision measurements of the neutron lifetime and decay asymmetries continue to test the framework of the Standard Model of particle physics and to probe for new physics beyond.  For the latter, experiments with improved precision on low energy beta-decay observables can be sensitive (via quantum loops) to new physics at energy scales comparable to and exceeding those probed directly by high energy colliders.  However, over the past decade, an outstanding discrepancy of $\sim$10 seconds (4 standard deviations) persists: the neutron lifetime measured in a beam (887.7 $\pm$ 2.2 s) is longer than that measured by counting surviving neutrons in a bottle (878.5 $\pm$ 0.8 s).  In this seminar, I describe the UCNtau experiment, which is designed to eliminate loss mechanisms present in previous bottle experiments by levitating polarized ultracold neutrons above the surface of a magnetic trap.  The asymmetric trap facilitates phase-space mixing, such that neutrons in quasi-stable orbits rapidly exit the trap.  An {\it in-situ} neutron detector operated in a multi-step counting scheme enables spectral monitoring and constrains systematic effects due to insufficient spectral cleaning and microphonic heating.  Using this approach, the lifetime reported, 877.7 $\pm$ 0.7 (stat) +0.3/-0.1 (sys) s [Science May 2018], is the first modern measurement of the lifetime that does not require corrections larger than the quoted uncertainties.

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