Session ML: Neutron Physics I: Physics with Neutron Beams

An Update on the BL2 In-Beam Neutron Lifetime Experiment and Discussion of Systematic Effects

Neutron beta decay is the simplest example of nuclear beta decay and is important to our understanding of weak processes, the unitarity of the CKM matrix in the Standard Model, Big Bang Nucleosynthesis models, solar physics, and the detection of reactor antineutrinos. The BL2 in-beam neutron lifetime experiment seeks to address a current discrepancy between the result of a previous NIST beam-based neutron lifetime experiment and recent ultra-cold neutron bottle lifetime experiments through careful study of possible systematic effects. This talk will address key systematic effects relevant to both the previous NIST beam lifetime experiment and BL2. An update on the status and outlook of the BL2 experiment will also be presented.

    

Detection Efficiencies in the BL2 In-Beam Neutron Lifetime Measurement

Precision measurements of neutron beta decay can provide answers to some of the most fundamental questions in particle physics, astrophysics and cosmology. Neutron beta decay, a semi-leptonic decay, is the simplest form of nuclear beta decay; therefore, it provides a clean test of the weak interaction of the Standard Model (SM). A precise measurement of the neutron lifetime and λ the ratio of axial vector and vector coupling constants of the weak interaction, allows for a determination of the Cabibbo-Kobayashi-Maskawa (CKM) matrix element V_ud that is free from nuclear structure effects. The SM predicts that the CKM matrix is unitary; therefore, the measurement of the neutron lifetime provides an important test of the SM. The neutron lifetime is also an important input parameter into early universe Big Bang nucleosynthesis calculations. The in-beam method of measuring the neutron lifetime requires the absolute counting of decay protons in a neutron beam of precisely known flux. Improvements in the neutron and proton detection systems as well as the use of a new analysis technique and apparatus upgrades allow for a rigorous re-examination of the systematic effects associated with this method. This work will discuss the results of new systematic studies regarding the detection efficiencies of protons and molecular hydrogen. New results from the simulation used for these studies will be discussed.

    

Progress on a Suite of Precision Measurements with the NIST Alpha-Gamma Device

The Alpha-Gamma device at the National Institute of Standards and Technology utilizes the interaction of neutrons with a totally absorbing $^{10}$B target to precisely measure the flux of a monochromatic neutron beam. This measurement provides a calibration of the $^{6}$Li(n,$\alpha$)$^{3}$H based flux monitor used in the NIST neutron lifetime experiment to better than 0.1\,\% and is also being utilized in novel, 0.2\,\% level, measurements of the $^{235}$U neutron-induced fission and $^6$Li neutron capture cross sections in an effort to provide systematically independent determinations of these important quantities. Each of these measurements requires ancillary efforts to characterize, for example, device solid angles, target characteristics, and alpha counting efficiencies. The results of recent and ongoing characterization measurements will be presented.   

Neutron Spin Rotation: Theoretical Predictions and Experimental Updates

The NSR Collaboration has performed experiments using slow neutron polarimetry to test parity violation in the hadronic weak interaction (HWI) and to search for exotic interactions in beyond-the-Standard Model physics. Updated theoretical predictions suggesting a new upper bound for the neutron rotary power in liquid ⁴He will be compared with the previous measurement of slow neutron spin rotation due to PV in HWI^[1]. We will also discuss theoretical motivations for an improved NSR search for an exotic axial vector interaction via light boson exchange^[2] and compare to bounds on exotic coupling constants measured using other experimental methods. Ongoing updates to both target systems of the polarimeter will be discussed. 

[1] H. E. Swanson et al., Phys. Rev. C 100 (2019)

[2] C. Haddock et al., Phys. Lett. B 783 (2018)   

Neutron Spin Rotation: Data Acquisition and Sensitivity Updates

The previous neutron spin rotation experiments conducted by the NSR Collaboration were able to obtain upper bounds of (2.1+/-8.3(stat.)+2.9/-0.2(sys.)) x10^-7 rad/m for liquid Helium [2] and (2.8+/-4.6(stat.)+/-4.0(sys.))x10^-5 rad/m for g_A² neutron coupling to matter [1]. In addition to new theoretical predictions and target system improvements, the NSR polarimeter has undergone several upgrades in its data acquisition software and instrumentation. These new systems will be used in an upcoming set of runs at the NIST NG-C beam line where significant systematic and statistical error reduction is expected. Details of the data acquisition and instrumentation and projections for statistical and systematic sensitivities will be discussed.

[1] Chris Haddock et al., Phys. Lett. B 783 (2018)

[2] H. E. Swanson et al., Phys. Rev. C 100 (2019)   

Measuring Higher Order Neutron-Silicon Structure Factors with Pendellösung Interferometry Using a Pulsed Beam

Dynamical diffraction gives rise to multiple waves inside perfect crystals when the incident wave nearly satisfies a Bragg scattering condition. The interference of these waves, called pendellösung, can be observed by "fringe-like" modulation of the intensity of the transmitted or diffracted beams exiting the crystal. Pendellösung interferometry can be used to precisely determine neutron-silicon structure factors, which may be used to investigate interactions Beyond the Standard Model, measure the internal structure of the neutron via the neutron charge radius, and provide information on thermal motion of the atoms in a lattice. While neutron-silicon structure factors have recently been measured for the (111), (220), and (400) reflections, quality data do not yet exist for the case of high-order reflections. Progress towards using the pulsed beam at the VULCAN beamline located at the Spallation Neutron Source run by Oak Ridge National Lab to measure the (333), (444), and (555) reflections simultaneously will be discussed. Leveraging the pulsed beam to measure multiple structure factors simultaneously will reduce some of the systematic uncertainties associated with the previous experiment. A successful measurement will allow for the study of anharmonic contributions, increase the precision of the determined neutron charge radius, and provide further constraints on an atomic length scale "fifth" force.   

Proton drift effects in beam neutron lifetime experiments

In beam neutron lifetime experiments conducted at ILL and NIST, the neutron beam passes through a quasi-Penning trap that confines neutron decay protons for subsequent counting. The trap features an axial magnetic field throughout and electric fields with axial and radial components near the ends of the trap. In general the trajectory of a charged particle in the adiabatic regime of a strong magnetic field is a helical motion around the magnetic field line coupled with a drift velocity perpendicular to the field line caused by ExB and field gradient effects. This drift mainly produces a slow azimuthal magnetron orbit about the trap axis. However breaking of azimuthal symmetry by the trap fields may cause radial drifts and a possible loss of protons from the trap. Results of recent analysis and simulations of these effects will be presented.