Session E14: Neutron Lifetime

The BL3 Beam Neutron Lifetime Experiment

Neutron beta decay is the simplest example of nuclear beta decay and can provide a test of the unitarity of the CKM matrix free from nuclear structure effects. The neutron lifetime is also important in Big Bang Nucleosynthesis models, solar physics, and the detection of reactor antineutrinos. However, there is a current large discrepancy between beam-based neutron lifetime experiments and recent ultra-cold neutron storage experiments. The goal of the BL3 Beam Neutron Lifetime Experiment is to improve the precision of beam-based experiments to the 0.3 s level while performing a thorough evaluation of potential systematic effects at the same level. This experiment will utilize a new, larger apparatus with a larger proton trap and detector; an improved neutron flux monitor; and an upgraded version of the alpha-gamma device that provides the absolute calibration of the neutron flux monitor. This talk will discuss the measurement technique used by the BL3 experiment as well as provide an overview and status update on the construction of the new apparatus, currently underway.   

The Analysis of Systematic Effects of 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.   

Monte Carlo Simulations in the BL3 Experiment

Neutron beta decay is a fundamental process in nuclear physics that provides sensitive means to uncover the details of the weak interaction. A precise measurement of the neutron lifetime, along with neutron beta decay correlations, can provide tests of CKM unitarity and is needed for Big Bang Nucleosynthesis calculations of the primordial $^4$He abundance, which correlates with other cosmological parameters. A next generation beam method experiment, BL3 at the NCNR, aims to improve the systematic precision of neutron lifetime measurements in order to resolve the discrepancy between beam and bottle method measurements. In BL3, the recoil protons from neutron beta decay are born in a quasi-Penning trap with magnetic and electric fields that transport them to a segmented silicon detector. The BL3 experiment will use higher neutron flux in a larger beam, which necessitates a new, large, pixelated proton detector. The BL3 collaboration is using a combination of simulation tools to assess systematic effects and the anticipated performance of the apparatus. We will present Monte Carlo simulations of neutron beam studies and decay proton transportation from the trap to the silicon detector.   

Alpha/Triton Detection in the BL3 Experiment

The BL3 apparatus is being constructed with the capability to measure the in-flight decay neutron lifetime with statistical uncertainty better than 1 s in less than one day. This will support a comprehensive study of systematic effects, which may be responsible for discrepancy with measurements using trapped ultra-cold neutrons. The alpha/triton collimation and detection system was redesigned in the neutron and alpha-gamma detectors to accommodate higher rates and a larger beam spot on the NIST NG-C neutron guide. The new geometry and symmetry offer near perfect neutron detection uniformity, advanced neutron beam diagnostics, and high enough detection rates to test a new singles-coincidence calibration mode.   

Optimization and Fabrication of Hyperboloid Apertures for the BL3 Experiment

The large-diameter neutron beam in BL3 poses new challenges for neutron detection uniformity in the BL3 experiment. Hyperboloid shaped apertures are easily manufactured using a wire EDM machine and offer a promising way of reducing systematic uncertainties in the mean neutron lifetime associated with neutron detection uniformity to below the goal level of 0.1 seconds. In this talk, I will discuss the optimization/design process and compare the alignment sensitivity/performance of these apertures to the more standard planar apertures used in previous experiments.   

Analysis of the latest UCNτ dataset for neutron lifetime measurement

Beta decay of a free neutron is the simplest form of "semi-leptonic" weak interaction and is free from nuclear structure effects. Despite the simplicity of the process, its lifetime measurement remains one of the most challenging measurements, bearing different results depending on the technique ("bottle" or "beam"). Another critical measurement from the decay is the correlation (A_o) between the neutron's initial spin and emitted electron's momentum. Neutron lifetime and axial coupling constant determined using A_o are inputs to determine the magnitude of the Cabibbo-Kobayashi-Maskawa (CKM) matrix element (V_ud), and provide a means to study physics beyond the Standard Model.

UCNτ is an experiment utilizing the ultracold neutron source [1] at LANL. The experiment consists of loading the neutrons in a magneto-gravitational trap and counting the neutrons surviving the beta decay using a ¹⁰B-coated ZnS scintillator. The experiment so far has provided the most precise neutron lifetime in the world with a value of 877.75 ± 0.28_stat + 0.22/−0.16_syst s [2]. With UCNτ being upgraded to UCNτ+, the data set from the experiment (2020-2022) is being analyzed to get the latest neutron lifetime results. In this contribution, I will discuss the analysis methodology for the data and explain the role of various systematics in the measurement.   

Status of UCNτ Experiment to Measure Neutron Lifetime

Determining the lifetime of a free neutron is an essential experimental observable for developing our understanding of particle physics and cosmology, as it plays a significant role in determining the abundances of light elements in the early universe and provides input for testing physics beyond the Standard Model. The UCNτ experiment has produced the world’s most precise measurements of τ_n, the free neutron lifetime, by storing ultracold neutrons (UCN) in a magneto-gravitational trap designed to remove relatively high-energy UCNs and avoid neutron-wall collisions. The most recent result from UCNτ specifies a lifetime of 877.75 ± 0.28 (stat) +0.22/-0.16 (sys) seconds where the limiting factor is statistical precision. The goal for the upgrade from UCNτ to UCNτ+ is to reduce the lifetime uncertainty to the ~0.1 second level. To achieve this precision, the neutron delivery system to the trap is being replaced with a more efficient system using an elevator concept. Instead of removing a small section of the array and guiding the neutrons up into the trap, the new delivery system will fill a bin above the trap. The bin is then moved through the array, adiabatically transporting the UCN into the trap. This talk will present the concept of the UCNτ+ experiment and show progress of the experimental work towards the upgrade.   

Status of the UCNProBe experiment

The free neutron lifetime is an active subject of interest due to differences in the observed lifetime between cold neutron beam experiments and trapped ultra-cold neutron (UCN) measurements. This difference is currently around the four sigma level. The UCNProBe experiment at Los Alamos will perform a beam experiment using UCNs by trapping the neutrons within a storage volume of deuterated scintillator and counting both the number of UCNs within the trap and number of decay electrons to within 0.1 percent precision. The experiment will be conducted in two iterations. The first will use a boron-coated dagger to count the UCNs, while the later version will use helium-3 gas instead. In this talk, we will present the design studies of the boron-coated dagger and discuss potential systematics and detection efficiencies for both iterations of the experiment.   

Effective field theory approach for radiative corrections to neutron decay

We develop a new top-down effective field theory approach for radiative corrections to the neutron decay. First, we match the Standard Model to the four-fermion effective field theory. To evaluate radiative corrections at scales of the neutron decay, we perform matching to the heavy-baryon chiral perturbation theory. For the vector coupling constant, we find an agreement with the traditional current-algebra approach at the one-loop level, improve on the resummation of next-to-leading logarithms, and provide an updated extraction of the Vud matrix element.   

Measurement of Proton's Energy Loss Through Thin Gold Film

The National Institute of Standards and Technology (NIST) aims to reduce the systematic uncertainties of the energy spectrum in the neutron's lifetime experiment, but the presence of any "dead" layer inside the detector affects the energy spectrum of low-energy ions. In order to take this factor into account, we conducted this experiment to determine energy loss spectra of proton beam within the Gold dead layer and compare this spectra with the Stopping Power (SP) of Gold from the NIST's database. To that end, we generated 50-200 keV energy proton beam in Gettysburg College's 200 keV Van de Graff proton accelerator and collected the proton counts received by the detector, containing Gold layer, for different corresponding energy channels using energy spectra measuring software. As a result, we had to evaporate gold onto different batches of circular glass slides, and Si detector within the Gold Evaporator chamber and determined the thickness of gold layers by utilizing UV-Vis Spectroscopy technology, and AFM Technology. Employing these data, we plotted SP of the Gold dead layer versus the kinetic energy of the proton beam using the thickness function of the Gold, and differentiated this plot with the graph of SP of Gold collected from the NIST database.