Bulletin of the American Physical Society
APS April Meeting 2018
Volume 63, Number 4
Saturday–Tuesday, April 14–17, 2018; Columbus, Ohio
Session R12: Electroweak Interactions |
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Sponsoring Units: DNP Chair: Hans Mumm, NIST Room: A222-223 |
Monday, April 16, 2018 10:45AM - 10:57AM |
R12.00001: Status of the BL2 beam measurement of the neutron lifetime Shannon Fogwell Hoogerheide Neutron beta decay is the simplest example of nuclear beta decay, and a precise value of the neutron lifetime is important for consistency tests of the Standard Model and Big Bang Nucleosynthesis models. A new measurement of the neutron lifetime, utilizing the beam method, is underway at the National Institute of Standards and Technology Center for Neutron Research with a projected uncertainty of 1 s. An overview of the beam method will be presented. The status of the experiment, including preliminary measurements, beam characteristics, and technical improvements, will be discussed. [Preview Abstract] |
Monday, April 16, 2018 10:57AM - 11:09AM |
R12.00002: The sensitivity and reach of UCN$\tau$ and how we get there Eric Fries There have been various measurements of the free neutron lifetime using either cold neutron beams, or ultracold neutrons (UCNs) stored in a trap. There is a ~$4\sigma$ discrepancy in measured lifetimes between the two methods. The UCN$\tau$ experiment uses an asymmetric magneto-gravitational trap to store neutrons, holds the neutrons for various times, and counts the remaining neutrons to measure the neutron lifetime. To date, UCN$\tau$ has acquired sufficient data to measure the neutron lifetime with a raw statistical uncertainty of about 0.35 s, and analysis of our 2016/2017 data set has established a current limit for our systematic uncertainty of 0.28 s. Moving forward, we expect to further reduce both the systematic and statistical uncertainties. We will present projections for how the systematic uncertainties can be better constrained, as well as discuss our plan for reducing the statistical uncertainty achievable during a run cycle. [Preview Abstract] |
Monday, April 16, 2018 11:09AM - 11:21AM |
R12.00003: A next generation neutron lifetime measurement based on UCN$\tau $ Alexander Saunders The UCN$\tau $ experiment measures the free neutron lifetime by in situ counting of surviving ultracold neutrons after different storage times in an asymmetric magneto-gravitational storage volume.~ This experiment has acquired sufficient data for a measurement of the neutron lifetime with a statistical uncertainty of about 0.35 s and has demonstrated a systematic uncertainty of 0.28 s; it is expected to ultimately reach a total uncertainty of about 0.2 s.~ To achieve even better precision, the leading sources of uncertainty, which include counting statistics, microphonic heating of the stored neutrons, evolution of the neutron population in phase space, counting rate-dependent effects, and interactions with the residual gas in the storage volume, must be addressed.~ In this talk, we will discuss how to reduce these sources of uncertainty to achieve a total uncertainty on the neutron lifetime well below 0.1 s in the second generation Tau2 experiment. [Preview Abstract] |
Monday, April 16, 2018 11:21AM - 11:33AM |
R12.00004: The UCN$\tau$ measurement of the free neutron lifetime: status report Chris Cude-Woods The UCN$\tau$ experiment measures the $\beta$ decay lifetime of the free neutron. Ultracold neutrons (UCN) are loaded into a 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 experiment completed its first production science run during the 2017 beam cycle at the Los Alamos Neutron Science Center, roughly quadrupling data taken during previous commissioning and engineering runs and leading to a statistical uncertainty well below 0.5 s. We will present the current status of the analysis process and expected total uncertainties in this data set. [Preview Abstract] |
Monday, April 16, 2018 11:33AM - 11:45AM |
R12.00005: A Precision Measurement of Unpolarized Neutron Beta Decay in the Nab Experiment Jason Fry The Nab experiment will make measurements of the electron-neutrino correlation parameter $a$ with a precision of $\delta a / a = 10^{-3}$ and the Fierz interference term $b$ to $\delta b = 3\times10^{-3}$ in unpolarized free neutron $\beta$ decay. These results aim to deliver an independent determination of the ratio $\lambda = G_A / G_V$ that will sensitively test CKM unitarity. Nab utilizes a novel, long asymmetric 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 under installation on the Fundamental Neutron Physics Beamline at the SNS at ORNL. We present an overview of the Nab experiment and updates on the spectrometer, magnetometry, and systematic effects. [Preview Abstract] |
Monday, April 16, 2018 11:45AM - 11:57AM |
R12.00006: The Measurement and Mitigation of Residual Beam Polarization for the Nab Experiment Chelsea Hendrus The Nab Experiment at 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 unwanted residual polarization in the incident neutron beam. The experimental approach to understanding this effect involves measuring this small polarization of the FnPB beam using a polarized $^{3}$He spin filter, and mitigating the measured polarization using newly designed Adiabatic Fast Passage (AFP) neutron spin flipper. [Preview Abstract] |
Monday, April 16, 2018 11:57AM - 12:09PM |
R12.00007: Detection systematics in the Nab experiment Leah Broussard The Nab experiment will perform precise measurements of neutron beta decay correlations to test our understanding of the electroweak interaction and look for hints of new physics missing from the Standard Model. The electron anti-neutrino correlation $a$ is reconstructed from the decay proton's time of flight and decay electron's energy, and the Fierz interference term $b$ is extracted by precisely measuring the shape of the beta spectrum. Reaching Nab's goal precision requires accurately characterizing systematics such as mechanisms for energy loss of the electrons and differences in the measured timing of the electron and proton events. Precision characterization of these effects can also be applied to a recent measurement of the beta spectrum of $^{45}$Ca or other nuclei using these detectors. This presentation will include an update on the detection system development and recent studies, a discussion of the detection systematics and their impact on the decay correlation measurements, and an overview of plans for their characterization. [Preview Abstract] |
Monday, April 16, 2018 12:09PM - 12:21PM |
R12.00008: Neutron-antineutron conversion to search for B-L violation Xinshuai Yan, Susan Gardner The observation of the violation of baryon number minus lepton number (B-L) would reveal the existence of physics beyond the Standard Model, and it would prove that fundamental Majorana dynamics exists. We propose searching for B-L violation through $n-\bar{n}$ conversion, in which the $|\Delta B|=2$ process is mediated by an external source. We construct the leading-mass-dimension, quark-level operators for $n-\bar{n}$ conversion since they differ from those for $n-\bar{n}$ oscillation, and we evaluate the sensitivity of possible scattering experiments to B-L violating parameters. Finally we show, under particular assumptions, how a limit on $n$-${\bar n}$ conversion can be interpreted as a limit on the neutron's Majorana mass. [Preview Abstract] |
Monday, April 16, 2018 12:21PM - 12:33PM |
R12.00009: Characterization of Noise Sources in the n3He Parity Violating Asymmetry Measurement Mark McCrea The goal of the n3He Experiment was to measure the parity-violating proton directional asymmetry relative to the initial neutron polarization in the reaction $\vec{n}+ ^3He \rightarrow p + T + 765\;\mathrm{keV}$ to a high precision. Data taking completed at the end of 2015 yielding a preliminary proton asymmetry of $(1\pm1)\times10^{−9}$. In addition to neutron counting statistics there are smaller contributions to the uncertainty in the final asymmetry due to electronics and other instrumental noise sources. I will present a characterization of this noise and algorithms to minimize its contribution to the uncertainty of the proton asymmetry. [Preview Abstract] |
Monday, April 16, 2018 12:33PM - 12:45PM |
R12.00010: Searching for Tensor Currents in the Weak Interaction Using $^{8}$Li $\beta$ Decay M.T. Burkey, G. Savard, R.E. Segel, J.A. Clark, J. Klimes, N.D. Scielzo, A.T. Gallant, K. Kolos, S.W. Padgett, B.S. Wang, T. Hirsh, R. Orford, E. Heckmaier, J. Pierce, L. Varriano, D. Burdette, S.T. Marley, G. Morgan, K.S. Sharma Precision beta-neutrino correlation measurements are often used as a broadband test for New Physics by limiting interaction contributions other than the vector-axial vector structure the Standard Model requires. We present the results of a pure Gamow-Teller $\beta-\nu$ correlation coefficient (a$_{\beta\nu}$) measurement via the beta decay of $^{8}$Li, which is sensitive to tensor currents. This data set was taken at Argonne National Lab with the Beta decay Paul Trap (BPT) and surpasses the statistics of our previous limit-defining $^{8}$Li experiment by an order of magnitude. With the analysis nearing its conclusion, we intend to push the low energy limit of a$_{\beta\nu}$’s relative uncertainty into the 0.1 percent range. [Preview Abstract] |
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