Bulletin of the American Physical Society
2021 Fall Meeting of the APS Division of Nuclear Physics
Volume 66, Number 8
Monday–Thursday, October 11–14, 2021; Virtual; Eastern Daylight Time
Session ML: BSM Searches in Fundamental Symmetries VIII: Neutrons |
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Chair: Amy Nicholson, UNC Room: Georgian |
Wednesday, October 13, 2021 4:00PM - 4:12PM |
ML.00001: The Nab Experiment: Examining Unpolarized Neutron Beta Decay Correlations Jason A Fry Neutron beta decay is one of the most fundamental processes in nuclear physics and provides sensitive means to uncover the details of the weak interaction. Neutron beta decay can evaluate the ratio of axial-vector to vector coupling constants in the standard model, λ = GA / GV, through multiple decay correlations. The Nab experiment will make measurements of the electron-neutrino correlation parameter a with a relative precision of δa / a ≈ 10-3 and the Fierz interference term b, to a few x10-3 in unpolarized free neutron beta decay. These results aim to deliver an independent determination of the ratio λ that will sensitively test CKM unitarity, as well as probe exotic electroweak scalar and tensor currents. Nab utilizes a novel, long asymmetric electromagnetic spectrometer that guides the decay products to two large area silicon detectors in order to precisely determine the electron energy and proton momentum. The Nab apparatus is finishing installation and beginning commissioning on the Fundamental Neutron Physics Beamline at the SNS at ORNL. We will present an overview and updates of the Nab experiment. |
Wednesday, October 13, 2021 4:12PM - 4:24PM |
ML.00002: Pulse Shape Discrimination for the Nab Experiment David Mathews, Leah J Broussard The Nab neutron beta decay correlation experiment will measure "a", the electron-neutrino correlation coefficient, and "b", the Fierz interference term. "a" serves as a test of CKM matrix unitarity while "b" places restrictions on the presence of scalar and tensor couplings in the weak interaction. These measurements will be done via detection of coincident protons and electrons with a pair of pixelated silicon detectors. The signals created by the incident particles vary in shape depending on parameters such as hit location within a pixel. Such variations can cause systematic shifts in timing and energy reconstruction. The Nab experiment precision goals necesitate understanding the mean timing bias to less than 1ns. A novel pulse shape discrimination technique has been developed utilizing GPUs to match waveforms to template shape functions to correct for biases in timing extraction from digitized signals. The effectiveness of this method at extracting proton hit locations from data collected at the University of Manitoba will be presented. |
Wednesday, October 13, 2021 4:24PM - 4:36PM |
ML.00003: Closed Loop Helium Gas Cooling System for the Nab Experiment Love Christie The Nab experiment at the Spallation Neutron Source will use an unpolarized neutron beam to measure the electron-neutrino correlation coefficient, 'a', and the Fierz interference term, 'b', to further our understanding of the weak interaction. The protons and electrons from neutron beta decay will be measured by pixelated silicon detectors. To minimize noise and optimize energy and timing resolution, the detector will be actively cooled to below 200K. Temperature regulation will be achieved by a closed loop helium gas cooling system. This talk will focus on the design, characterization, and performance of this system. |
Wednesday, October 13, 2021 4:36PM - 4:48PM |
ML.00004: A Detector Testing Chamber for the Nab Experiment Michelle H Gervais, Leah J Broussard The Nab experiment is studying the decay electrons and protons from neutron beta decay in the search for BSM physics. It will use pixelated silicon detectors to detect these decay particles. It is important to characterize timing and energy response of these Nab detectors and the impact of charge sharing among detector pixels to understand the systematic errors. This includes the timing response to the decay protons and electrons, which must be measured under controlled conditions. We will use a proton beam at the University of Manitoba to test the detector response to protons as a function of energy, position, and angle of impact. We will also study multipixel events, charge sharing, individual channel trigger thresholds and their impact on total measured energy. In addition, how well we can recover particle energies in multipixel events. |
Wednesday, October 13, 2021 4:48PM - 5:00PM |
ML.00005: Update on the BL2 Experiment: An In-Beam Measurement of the Neutron Lifetime Jimmy Caylor Neutron beta decay is the simplest example of semi-leptonic decay. The neutron lifetime provides an important test of unitarity and consistency of the Standard Model. The neutron lifetime is also the largest uncertainty in Big Bang Nucleosynthesis calculations of light element abundance. A precise measurement of the neutron lifetime and λ, the ratio of axial vector and vector coupling constants of the weak interaction, allow for a determination of the CKM matrix element Vud that is free from nuclear structure effects. A new measurement of the neutron lifetime using the in-beam method is ongoing at the NIST Center for Neutron Research. This method 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 should allow for a thorough investigation of major systemic effects. The experimental status, systematic tests, analysis techniques and early data will be presented. |
Wednesday, October 13, 2021 5:00PM - 5:12PM |
ML.00006: Refining the Limit on Depolarization Loss Rate in the UCNτ Magneto-gravitational Trap Adam T Holley By counting the number of polarized, very low energy "ultracold" neutrons (UCN) surviving confinement in a magnetic plus gravitational potential, the UCNτ collaboration has completed a measurement of the free neutron lifetime τn with 0.04% relative precision. An advantage of our approach is that the expected UCN loss rate due to interaction with the magnetic part of the trapping potential is negligible relative to the ∼0.01% precision targeted for probes of beyond the Standard Model physics. The actual depolarization rate in the as-built apparatus is empirically constrained via measurement of the trap lifetime as a function of the polarization-preserving "holding field" strength. We will present results from our spin-tracking simulation effort that incorporate a higher-fidelity field model in order to study the scaling of depolarization rates for smaller holding field values, better constraining the nominal loss rate. |
Wednesday, October 13, 2021 5:12PM - 5:24PM |
ML.00007: Preliminary Implementation of a Movable Miniature UCN Detector as a UCN Energy Spectrometer Jin Ha Choi The UCN$\tau$ experiment at the Los Alamos Neutron Science Center measures the mean lifetime of free neutrons by loading ultracold neutrons (UCN) into a magneto-gravitational trap and counting remaining UCN after waiting various storage times. The energy of a UCN determines the maximum height it can reach, thus the energy spectrum of the UCN affects the number of UCN loaded into the trap as well as the relative counts on the monitor detectors that are placed at different heights. |
Wednesday, October 13, 2021 5:24PM - 5:36PM |
ML.00008: The Next Generation of Neutron Lifetime Experiments at Los Alamos Neutron Science Center Rifet Musedinovic UCN$\tau +$, now in development at the Los Alamos Neutron Science Center, is an upgrade of the UCN$\tau$ experiment. Recently the UCN$\tau$ experiment reported the most precise measurement of the free neutron lifetime using trapped ultracold neutrons (UCN), specifying the lifetime with an uncertainty of 0.041% (0.36 s). High precision neutron decay data determines the coupling constants for the weak interaction of the nucleon without nuclear structure-dependent effects, with the current precision from UCN$\tau$ now within a factor of two of the limiting theoretical uncertainties due to radiative corrections. Given the sensitivity precision neutron decay measurements offer to new physics (through unitarity tests, for example), an upgrade to UCN$\tau$ with a factor of two or greater improvement to the precision is greatly motivated. The UCN$\tau +$ upgrade focuses on increasing the loaded number of UCN in the magneto-gravitational trap and improving the high rate performance of the UCN$\tau$ \textit{in situ} detector. We provide an overview of the UCN$\tau +$ project, focusing on recent work modeling an adiabatic transport system to load the trap. In addition, we explore the transport parameters to optimize the gains obtained from cooling over-threshold UCN. |
Wednesday, October 13, 2021 5:36PM - 5:48PM |
ML.00009: Measurement of the Free Neutron Lifetime using the Neutron Spectrometer on NASA's Lunar Prospector Mission Jack Wilson, David J Lawrence, Patrick N Peplowski, Vincent R Eke, Jacob A Kegerreis Knowledge of the free neutron lifetime, τn, is important as, along with other neutron decay parameters, it enables the testing of the unitarity of the CKM matrix in the Standard Model. Additionally, uncertainties in τn dominate those in predictions of primordial He abundance from Big Bang nucleosynthesis. Historically, there have been two classes of high-precision experiments that measure τn. However, a 4.5 σ disagreement exists between the results of the two techniques. We recently demonstrated an alternative measurement method using space-based neutron spectroscopy measurements with data from NASA’s MESSENGER mission. However, due to the constraints of the mission this measurement had significant uncertainties. Here, we use data from the Lunar Prospector Neutron Spectrometer to make the second space-based measurement of the free neutron lifetime finding τn = 887 ± 15 s. Improvements in modeling enabled the reduction of the systematic uncertainty from 70 s on the previous space-based lifetime measurement to 7 s here. This modeling moves space-based neutron lifetime measurement towards the ultimate goal of reducing the magnitude of the systematics on a future space-measurement to the level of those seen in laboratory-based experiments. In this talk, we will discuss the improvements made to the space-based technique, and the statistical and systematic errors of the measurement. |
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