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# The physics of ultracold neutrons and Fierz interference in beta decay

## Citation

Hickerson, Kevin Peter (2013) The physics of ultracold neutrons and Fierz interference in beta decay. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechTHESIS:11022012-135115177

## Abstract

In the first component of this thesis, we investigate the physics of ultracold neutrons (UCN). UCN are neutrons so cold they can be stored inside of material, magnetic and gravitational bottles. Using this property we use UCN nonimaging optics to design a type of reflector that directs UCN upward in to vertical paths. Next we examine UCN passing through thin, multilayered foils. In the remaining sections we investigate the so-called Fierz interference term of free neutron beta decay, denoted $b_n$. It is theorized that contributions to scalar and tensor interactions from physics beyond the Standard Model could be detectable in the spectrum of neutron beta decay, manifest as a nonzero value for $b_n$. We investigate three techniques for measuring $b_n$. The first is to use the primordial helium abundance fraction and compare that to predictive Big Bang nucleosynthesis calculations. Second we extract $b_n$ from the spectral shape generated by the 2010 data set of the UCNA experiment. Third, we discuss progress toward constructing the UCNb experimental prototype. We present the design of this new experiment that uses the UCN source at LANSCE for measuring $b_n$, in which UCN are guided into a shielded $4\pi$ calorimeter where they are stored and decay. From Big Bang nucleosynthesis we can place the limit $0.021 < b_n < 0.277$ (90\% {C.L.}) on the neutron Fierz term and from the UCNA 2010 data we set $-0.044 < b_n < 0.218$ (90\% {C.L.}).

Item Type: Thesis (Dissertation (Ph.D.)) Ultracold neutrons, Fierz Interference California Institute of Technology Physics, Mathematics and Astronomy Physics Public (worldwide access) Filippone, Bradley W. Filippone, Bradley W. (chair)Hirata, Christopher M.Golwala, SunilLibbrecht, Kenneth GeorgeWise, Mark B. 9 October 2012 CaltechTHESIS:11022012-135115177 http://resolver.caltech.edu/CaltechTHESIS:11022012-135115177 No commercial reproduction, distribution, display or performance rights in this work are provided. 7250 CaltechTHESIS Kevin Hickerson 10 Dec 2012 21:42 09 Feb 2013 00:10

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