Bulletin of the American Physical Society
APS April Meeting 2019
Volume 64, Number 3
Saturday–Tuesday, April 13–16, 2019; Denver, Colorado
Session C14: Mini-Symposium:The Neutron Lifetime Anomaly - current statusFocus Session
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Sponsoring Units: DNP Chair: Nadia Fomin, University of Tennessee Room: Sheraton Plaza Court 3 |
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Saturday, April 13, 2019 1:30PM - 2:06PM |
C14.00001: The state of the neutron lifetime Invited Speaker: Shannon Fogwell Hoogerheide Neutron beta decay is the simplest example of nuclear beta decay and is crucial in our understanding of weak processes. The neutron lifetime, when combined with other neutron decay parameters, provides a test of the unitarity of the CKM matrix in the Standard Model. The value of the neutron lifetime is also an important input in Big Bang Nucleosynthesis models. Additionally, the value of the neutron lifetime is relevant in other areas including solar physics and the detection of reactor antineutrinos. Two main methods have been utilized to measure the neutron lifetime: the “bottle” method and the “beam” method. In the bottle method, ultracold neutrons are confined in a material and/or magnetic trap. After varying lengths of storage time, the number of neutrons remaining in the trap are counted. In the beam method a cold neutron beam is passed through a fiducial volume. The absolute neutron beam flux is measured, as well as the absolute number of decay particles (protons or electrons) resulting from neutron decay inside the fiducial volume. Despite the increasing precision of the individual methods, there has been a persistent 4 σ discrepancy between the bottle average value and the beam average value for the neutron lifetime. An overview of the measurement methods, including relevant challenges and planned/ongoing improvements, will be given. A variety of possible explanations for the discrepancy, ranging from experimental errors to new physics will be discussed. |
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Saturday, April 13, 2019 2:06PM - 2:18PM |
C14.00002: The BL2 Experiment: An In-Beam Measurement of the Neutron Lifetime Jimmy Caylor Neutron beta decay is the simplest example of semi-leptonic decay. 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. 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 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 beta 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 permit an overall uncertainty of 1s. The experimental status, technical improvements, analysis techniques and early data will be presented. |
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Saturday, April 13, 2019 2:18PM - 2:30PM |
C14.00003: Precision Measurement of Neutron Flux using Alpha-Gamma Evan R Adamek, Geoffrey L Greene, Nadia Fomin, Maynard Dewey, David M Gilliam, Shannon M Fogwell Hoogerheide, Hans P Mumm, Jeffrey Scott Nico, William Michael Snow A high precision measurement of cold neutron beam flux is a vital component of the beam method of neutron lifetime measurement used by the BL2 experiment. This measurement is accomplished through use of the Alpha-Gamma device. A pair of $^{10}$B targets are successively placed in the beam path such that the alpha and gamma production from the absorption determines the absolute neutron flux. The Alpha-Gamma device has been used in the calibration of several iterations of flux monitor used in BL2 to better than 0.1% precision. The results of recent measurements will be presented, and progress on the next generation of the device will be discussed. |
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Saturday, April 13, 2019 2:30PM - 2:42PM |
C14.00004: The UCNτ measurement of the free neutron lifetime: overview and status report Christopher Cude-Wood The UCNτ experiment measures the mean decay lifetime of the free neutron. Ultracold neutrons (UCN) from the Los Alamos ultracold neutron facility are loaded into an asymmetric trap wherein they are confined by gravity and an array of permanent magnets, and, after varying storage times, the surviving UCN are counted in-situ. The recent installation of a preconditioning storage volume has improved the ability to normalize the number of neutrons loaded into the trap for each data run by partially removing the effect of the pulsed nature of the Los Alamos UCN source. The experiment has completed acquiring production data during two annual accelerator run cycles, leading to a statistical uncertainty on the neutron lifetime well below 0.5 s. We will present the status of the analysis and the ongoing multiyear production run. |
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Saturday, April 13, 2019 2:42PM - 2:54PM |
C14.00005: Simulating the UCN$\tau$ Neutron Lifetime Experiment Francisco M Gonzalez The UCN$\tau$ experiment at Los Alamos National Laboratory measures the neutron lifetime by storing ultracold neutrons (UCNs) in a magneto-gravitational trap for variable holding times, which can be longer than the neutron’s lifetime. Systematic effects can occur due to changes in the UCN phase space distribution between different holding times, potentially exposing neutrons to loss mechanisms besides the fundamental neutron decay rate, or changing the detection efficiency of UCN in the trap. In order to minimize and understand possible sources of loss, we have utilized a Monte Carlo simulation of UCN trajectories. Additionally, this simulation allows us to model the UCN capture efficiency of the in-situ dagger detector. The simulation uses Indiana University’s Big Red II supercomputer to symplectically integrate neutrons in a magnetic potential derived from an analytic expression for the trap’s field. By modifying characteristics of the simulated trap, such as heating amount or cleaning height, we can look for effects on the measured lifetime. We will present results of these simulations as part of an effort to reduce UCN$\tau$’s total uncertainty to about 0.2 seconds. |
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Saturday, April 13, 2019 2:54PM - 3:06PM |
C14.00006: Measurement of systematic effects in the UCN$\tau$ experiment Eric M Fries One of the most important open questions about the fundamental properties of the neutron is the free neutron lifetime $(\tau_n).$ Experimental measures of $\tau_n$ can be broadly described in one of two ways: either a measurement using a cold neutron beam, or a measurement using ultracold neutrons (UCN) stored in a trap. There is a $\sim4\sigma$ discrepancy in measured $\tau_n$ between the two methods. The UCN$\tau$ experiment at Los Alamos Neutron Science Center fills an asymmetric magneto-gravitational trap with UCN, and then counts the surviving UCN after various holding times to measure $\tau_n.$ The UCN$\tau$ collaboration has published a systematic uncertainty of $^{+0.4}_{-0.2}$ s based on the analysis of our 2016-2017 data set. The effect of the depolarization of UCN is measured by varying the magnetic holding field magnitude. The effects of microphonic heating of trapped UCN and of insufficient cleaning of UCN with energies above the trapping potential are measured by detecting surviving high-energy UCN. The effect of phase space evolution of UCN is measured by comparing the time distributions of the surviving UCN after various holding times. We will present details of how these systematic effects are measured and projections of how they will be better constrained. |
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