Session LJ: Mini-Symposium on Fundamental Neutron Physics V

The measurement of neutron beta decay observables with the Nab spectrometer

Nab, an experiment that allows studying unpolarized neutron beta decay at the Spallation Neutron Source at Oak Ridge National Lab, aims to determine a, the neutrino-electron correlation coefficient, and b, the Fierz interference term, with high precision. Such measurements provide opportunities to search for evidence of extensions to the Standard Model. Nab is presently being constructed, with beam readiness planned for early 2019. I will discuss motivation and design, the planned modes of operation, and the performance of its components.   

Magnetometry Methodology of the Nab Experiment

The Nab experiment aims to measure the neutron beta decay electron-neutrino correlation coefficient $a$   to a relative uncertainty of  $10^{-3}$  in order to extract $ \lambda$, the ratio of axial to vector coupling constant, at roughly the same precision level as the vector coupling determined from superallowed decays. An additional measurement of $b$ with an uncertainty of $3 \times 10^{-3}$ would provide a sensitivity that could access new physics beyond the standard model. Nab uses an asymmetric magnetic spectrometer and two large-area highly segmented Si detectors to extract $a$ from the proton momentum and electron energy and $b$ from the electron energy spectrum. The design of the magnet spectrometer aligns the proton momentum perpendicularly to the Si detectors. Our measurement of the proton momentum depends strongly on the path length of the charged particle in the spectrometer, so a detailed mapping and analysis of the magnetic field is needed. This talk will discuss our method of mapping the magnetic field within the spectrometer and how the analysis of this field influences our measurement of $a$. 

Nab Si detector systematics, response and calibration measurements

The Nab experiment at the Spallation Neutron Source (SNS) will precisely measure the electron-neutrino correlation parameter and the Fierz interference term in neutron beta decay.  Nab uses a novel field expansion spectrometer to measure electron energy and proton momentum.  At either end of the spectrometer, particles are detected with a pixelated Si detector.  Several important systematic uncertainties depend on a precise understanding of detector response.  Nab's precision goals require $10^{-4}$ level precision in detector energy, as well as $\le$3 ns timing precision.  Preliminary detector testing has been done at Los Alamos National Laboratory and the University of Manitoba.  In addition to a discussion of Nab's detector systematics, an overview of planned detector calibration, as well as linearity and timing tests, to be done at the SNS, will be presented.   

Effect of Beam Polarization in the Nab Experiment at the SNS

The Nab Experiment on the Fundamental Physics Beamline (FnPB) at the Spallation Neutron Source (SNS) aims to precisely measure the electron-neutrino correlation parameter \textit{a},  and the Fierz interference term \textit{b}, associated with the beta decay of free neutrons. This measurement provides a cross-check and independent measurement of  $\lambda$, the ratio of vector to axial-vector coupling constants in the Standard Model. A potential source of systematic error in this experiment stems from residual polarization in the incident neutron beam. Whether or not such a polarization exists, and its possible size, is unknown.  Estimating the degree to which this polarization could affect the outcome of the experiment requires the integration of neutron beam polarization into ongoing studies of the systematic errors associated with the Nab Experiment. To further verify the extent of this effect the polarization of the FnPB can be measured using a polarized $^3$He spin filter and an Adiabatic Fast Passage Spin Flipper.   

Online Analysis Tools for the Nab Experiment

The Nab experiment is measuring the electron-neutrino correlation term, along with the Fierz interference term, within the theory of neutron beta decay. The electron-neutrino correlation parameter will be measured with a relative uncertainty of $10^{-3}$, and the Fierz interference term with an uncertainty of $\sim3\times10^{-3}$.  Data collection will begin in the Spring of 2019 at the fundamental physics beamline of ORNL's Spallation Neutron Source. Two segmented, large area, Si detectors will be used with a total of 254 pixels instrumented. Amongst the selection of analysis tools being developed, two will be presented. A method for identifying potential pile-up and accidental signals will be discussed, this being vital for energy reconstruction. In this method, a trapezoidal filter has been optimized for the task and a post-filter identification algorithm developed. Additionally, an online extraction of the electron-neutrino correlation parameter is desirable, but difficult given the computational intensity of a full analysis. A proposed analysis, using a Geant4 simulation to characterize the spectrometer's response with respect to available observables, will be presented.   

Precision Measurement of the Coherent Scattering Length of n-$^{4}$He Using Neutron Interferometry

Our recent work measured the coherent scattering length ($b_{c}$) of n-$^{4}$He to $10^{-3}$ relative precision, a factor of $10\times$ improvement over previous measurements. Neutron interferometry is used for precision measurements of scattering lengths for a variety of isotopes. Examples include the ($b_{c}$) of $^{1}$H, $^{2}$H, $^{3}$He and the incoherent scattering length of $^{3}$He. Neutron scattering lengths of light nuclei provide useful tests of nuclear potential models and can serve as inputs for nuclear effective field theories. We used a monolithic, perfect-silicon neutron interferometer which splits the wave-function of a single neutron, via Bragg diffraction, into two coherent separated paths and a phase shift, directly proportional to $b_{c}$, is introduced by the $^{4}$He sample. We will report our result and describe our method. This work was performed at the NIST Center for Neutron Research (NCNR) and is supported by the National Science Foundation.   

Precision Measurement of the Neutron-Electron Scattering Length

An update on the effort to measure the neutron-electron scattering length at the 0.5% level will be provided.  The experiment probes the Q²-dependence of pendellösung interference in silicon and germanium.  The pendellösung phase shift for three different Bragg conditions per material at a fixed temperature allows for the separation of neutron-electron scattering from the Debye-Waller factor.  The data may also be used to place competitive limits on the strength of an angstrom-scale fifth force.