27 research outputs found
Determination of the Axial-Vector Weak Coupling Constant with Ultracold Neutrons
A precise measurement of the neutron decay -asymmetry has been
carried out using polarized ultracold neutrons (UCN) from the pulsed spallation
UCN source at the Los Alamos Neutron Science Center (LANSCE). Combining data
obtained in 2008 and 2009, we report , from which we determine the ratio of the
axial-vector to vector weak coupling of the nucleon .Comment: 5 pages, 2 figure
First direct constraints on Fierz interference in free neutron decay
Precision measurements of free neutron -decay have been used to
precisely constrain our understanding of the weak interaction. However the
neutron Fierz interference term , which is particularly sensitive to
Beyond-Standard-Model tensor currents at the TeV scale, has thus far eluded
measurement. Here we report the first direct constraints on this term, finding
,
consistent with the Standard Model. The uncertainty is dominated by absolute
energy reconstruction and the linearity of the beta spectrometer energy
response
New result for the neutron -asymmetry parameter from UCNA
The neutron -decay asymmetry parameter defines the correlation
between the spin of the neutron and the momentum of the emitted electron, which
determines , the ratio of the axial-vector to
vector weak coupling constants. The UCNA Experiment, located at the Ultracold
Neutron facility at the Los Alamos Neutron Science Center, is the first to
measure such a correlation coefficient using ultracold neutrons (UCN).
Following improvements to the systematic uncertainties and increased
statistics, we report the new result which yields . Combination with the previous UCNA result and
accounting for correlated systematic uncertainties produces
and .Comment: 9 pages, 7 figures, updated to as-published versio
Search for neutron dark decay: n → χ + e⁺e⁻
In January, 2018, Fornal and Grinstein proposed that a previously unobserved neutron decay branch to a dark matter particle (χ) could account for the discrepancy in the neutron lifetime observed in two different types of experiments. One of the possible final states discussed includes a single χ along with an e⁺e⁻ pair. We use data from the UCNA (Ultracold Neutron Asymmetry) experiment to set limits on this decay channel. Coincident electron-like events are detected with ∼ 4π acceptance using a pair of detectors that observe a volume of stored Ultracold Neutrons (UCNs). We use the timing information of coincidence events to select candidate dark sector particle decays by applying a timing calibration and selecting events within a physically-forbidden timing region for conventional n → p + e⁻ + ν̅_e decays. The summed kinetic energy (E_(e⁺e⁻)) from such events is reconstructed and used to set limits, as a function of the χ mass, on the branching fraction for this decay channel
Final results for the neutron β-asymmetry parameter A₀ from the UCNA experiment
The UCNA experiment was designed to measure the neutron β-asymmetry parameter A0 using polarized ultracold neutrons (UCN). UCN produced via downscattering in solid deuterium were polarized via transport through a 7 T magnetic field, and then directed to a 1 T solenoidal electron spectrometer, where the decay electrons were detected in electron detector packages located on the two ends of the spectrometer. A value for A0 was then extracted from the asymmetry in the numbers of counts in the two detector packages. We summarize all of the results from the UCNA experiment, obtained during run periods in 2007, 2008–2009, 2010, and 2011–2013, which ultimately culminated in a 0.67% precision result for A₀