UCNtau research project
The UCNtau experiment measures the neutron lifetime using magnetically trapped ultracold neutrons (UCN) at the Los Alamos National Laboratory. In the UCNtau apparatus, UCN are magnetically levitated by an array of permanent magnets and confined by earth gravitational fields. The large volume of UCNtau, its asymmetrical construction, and the use of active neutron counting in-situ make it possible to reach a precise lifetime determination on the level of 10-4, which is needed to test the unitarity of the CKM matrix and probe physics beyond the standard model.
UCN (with kinetic energies < 50 neV) are confined in this Magneto-Gravitational (MG) trap. Magnetic trapping techniques exploit the repulsive force experienced by UCN polarized in the weak-field seeking state, F=μ ⋅ ∇B, effectively eliminating the dominant sources of loss from UCN storage experiment. The experiment employs the "bottle" technique to store neutrons inside the trap for a duration and then count the number of surviving neutrons. After 10 years of continuous improvement, the current results are the most precise experiments to date.
PhD student, Frank Gonzales, performed one of the three independent analyses of the high-statistics 2017-2018 dataset. The 0.039% precision of this result now approaches the level of theoretical uncertainty in view of the recently-revisited inner radiative corrections in beta decay and its uncertainty. This represents a crucial step towards realizing competitive tests of the standard model, through CKM unitarity, using neutron beta-decay.
We have yet to reach the ultimate limit of this apparatus for precision neutron lifetime measurement; the planned upgrade, UCNtau+, will allow us to boost the neutron counting statistics by an order of magnitude. In parallel, several systematic uncertainties were studied in detail during the 2019 and 2020 LANL run cycles. We inserted a high-pass UCN energy filter to better quantify possible systematics from marginally-trapped UCN. We continue to improve the multilayer ZnS scintillator UCN detector and tested a faster prototype LYSO-based scintillator detector to reduce the rate-dependent systematic effects.