Precision Measurement of the Neutron Lifetime

Citation

Fries, Eric Michael (2022) Precision Measurement of the Neutron Lifetime. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/qr2k-7b09. https://resolver.caltech.edu/CaltechTHESIS:08292021-214153213

Abstract

The neutron lifetime plays a critical role in Big Bang nucleosynthesis (BBN) calculations, and measurements of the neutron lifetime can also be used to probe the unitarity of the Cabibbo-Kobayashi-Maskawa quark weak mixing matrix. Most experiments that measure the neutron lifetime fall into two classes: "bottle" and "beam" experiments. A bottle experiment stores neutrons in a bottle and counts the number of neutrons that do not decay. A beam experiment counts the decay products of a beam of neutrons that passes through an electrostatic trap. An unresolved ≈ 4σ difference remains between the current global averages of measurements of the neutron lifetime using the bottle method and measurements using the beam method. This difference is the dominant uncertainty in BBN calculations of the helium mass fraction in the early universe. The UCNτ experiment is a bottle experiment which uses a magneto-gravitational trap to store ultra-cold neutrons (UCN) without any physical interactions between the UCN and the walls of the trap. The UCN that do not decay are counted with an in situ detector that is lowered into the trap. These two features stand in contrast to most past bottle experiments, which had to make significant corrections to the extracted lifetime to account for losses of neutrons due to material interactions with walls and losses while removing the neutrons from the bottle in order to be counted. This thesis will present an analysis of the 2017-2018 UCNτ data set that extracted a value for the neutron lifetime of 877.79 ± 0.27 (stat.) ^+0.19_-0.12 (sys.) s. This measurement has an uncertainty of roughly half of the current global average for the neutron lifetime.

Thesis Files