Bulletin of the American Physical Society
2013 Fall Meeting of the APS Division of Nuclear Physics
Volume 58, Number 13
Wednesday–Saturday, October 23–26, 2013; Newport News, Virginia
Session HE: Mini-Symposium on Fundamental Symmetries with Neutrons II |
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Chair: Nadia Fomin, University of Tennessee Room: Grand Ballroom V |
Friday, October 25, 2013 8:30AM - 8:42AM |
HE.00001: Framework for Maximum Likelihood Analysis of Neutron $\beta$ Decay Observables B. Plaster, S. Gardner We assess the ability of future neutron $\beta$ decay measurements of up to ${\cal O}(10^{-4})$ precision to falsify the standard model, particularly the $V-A$ law, and to identify the dynamics beyond it. To do this, we employ a maximum likelihood statistical framework which incorporates both experimental and theoretical uncertainties. Using illustrative combined global fits to Monte Carlo pseudodata, we also quantify the importance of experimental measurements of the energy dependence of the angular correlation coefficients as input to such efforts, and we determine the precision to which ill-known ``second-class'' hadronic matrix elements must be determined in order to exact such tests. [Preview Abstract] |
Friday, October 25, 2013 8:42AM - 8:54AM |
HE.00002: aCORN:-the electron-neutrino correlation coefficient in free neutron decay Brian Collett The decay of the free neutron provides a testing ground for the Standard Model without complications from nuclear structure. The electron-antineutrino angular correlation coefficient, ``$a$,'' is the least well determined of the angular correlation coefficients that have been precisely measured. An improved measurement of ``$a$'' will refine the value of the axial-vector coupling constant. The aCORN experiment aims to measure ``$a$'' to a relative accuracy of \textless\ 1{\%} using a novel method that does not depend on precise measurement of proton recoil energy. The apparatus features electron and proton detectors placed at opposite ends of a 3m long vacuum vessel in an axial magnetic field. An electrostatic mirror around the decay region directs protons to the proton detector in two populations that can be distinguished by time of flight. The value of ``$a$'' is proportional to the asymmetry between these populations. The experiment is currently operating on the NG-6 beamline at NIST. An overview of the method and experiment design will be presented. [Preview Abstract] |
Friday, October 25, 2013 8:54AM - 9:06AM |
HE.00003: aCORN Data Analysis and Preliminary Results Guillaume Darius After a thorough commissioning period during which, among other things, the magnetic field was tuned, the collimator assembly was precisely aligned and the proton detector and beta spectrometer were tested and characterized, aCORN is now taking quality data that will be used for the determination of little ``a,'' the electron-neutrino angular correlation coefficient. In this talk, I will briefly present the data acquisition system and data processing. I will then discuss the latest data, describe our analysis thus far, and show preliminary results. [Preview Abstract] |
Friday, October 25, 2013 9:06AM - 9:18AM |
HE.00004: Status of the UCNA Experiment David Phillips The UCNA experiment at the Los Alamos Neutron Science Center (LANSCE) is the only experiment to ever perform a measurement of the neutron $\beta$-decay asymmetry parameter $A(E)$ using ultracold neutrons (UCN). UCN are produced from a pulsed spallation solid deuterium source coupled to the 800 MeV proton beam at LANSCE. The UCN spin states are selected via a 7 T polarizing field and an adiabatic fast passage spin flipper. The polarized UCN are then transported to a 1 T 2$\times$2$\pi$ spectrometer where the emitted electrons are measured from a sample with density of order $\sim$1 UCN/cm$^{3}$ (since 2008 running). In the Standard Model, the leading order value of $A(E)$, $A_0$, is a function of the axial-vector to vector coupling ratio $\lambda \equiv g_{\rm A}/g_{\rm V}$, providing complementary data to the physics probed by measurements of the neutron lifetime $\tau_{\rm n}$. When taken together with $\tau_{\rm n}$, measurements of the beta decay asymmetry permit a nuclear structure independent determination of the CKM matrix element $V_{\rm ud}$. This talk presents an overview of the UCNA experiment, the analysis status of our 2011/2012 $\&$ 2012/2013 datasets, and the path forward. [Preview Abstract] |
Friday, October 25, 2013 9:18AM - 9:30AM |
HE.00005: Limits on Fierz Interference for the free neutron from the UCNA experiment Kevin Hickerson We report on limits on the Fierz interference term for the free neutron, $b_n$, using ultracold neutrons (UCN) as measured using the UCNA experiment at the Los Alamos Neutron Science Center. UCNA was designed to measure the energy dependence of the electron beta decay asymmetry using polarized neutrons. Limits on the Fierz term test consistency with the Standard Model (in which $b_n$ is negligibly small) and thus serves as a test for new physics. The $b_n$ parameter can skew the peak of the beta decay spectrum as well as modify the energy dependence of the asymmetry, $A(E)$. Using the same data used to measure $A$, we can simultaneously place limits on $b_n$, with only a partial decrease in sensitivity to $A$. This dual fit allows us to limit the effects of systematic errors, such as detector linearity and energy dependent detector efficiency. We will present results from the 2010 data, as well as the status of analysis of the data collected from the 2011-2013 run cycles. [Preview Abstract] |
Friday, October 25, 2013 9:30AM - 9:42AM |
HE.00006: Radiative Beta Decay for Studies of CP Violation Susan Gardner, Daheng He A triple-product correlation in the radiative $\beta$ decay rate of neutrons or of nuclei, characterized by the kinematical variable $\xi\equiv (\mathbf{l}_\nu\times\mathbf{l}_e)\cdot\mathbf{k}$, where, e.g., $n(p) \rightarrow p(p') + e^-(l_e) + \overline{\nu}_e(l_\nu) + \gamma(k)$, can be generated by the pseudo-Chern-Simons term found by Harvey, Hill, and Hill as a consequence of the baryon vector current anomaly and SU(2)$_{L}$$\times$U(1)$_Y$ gauge invariance at low energies. The correlation probes the imaginary part of its coupling constant, so that its observation at anticipated levels of sensitivity would reflect the presence of sources of CP violation beyond the standard model. We compute the size of the asymmetry in $n\to p e^- \bar\nu_e \gamma$ decay in chiral effective theory, compare it with the computed background from standard-model final-state interactions, and consider the new physics scenarios which would be limited by its experimental study. [Preview Abstract] |
Friday, October 25, 2013 9:42AM - 9:54AM |
HE.00007: Measurement of radiative neutron decay Matthew Bales By the simultaneous detection of a prompt photon and electron followed by a delayed proton, the RDK II experiment is able to reliably identify radiative neutron decays relative to non-radiative neutron decays in the photon energy range of approximately 0.3 keV to the 782 keV endpoint. RDK II will measure both the radiative branching ratio as well as the energy spectrum of emitted photons. The experimental apparatus consists of a superconducting solenoid magnet, which guides charged particles away from a cold neutron beam to a surface barrier detector located off the neutron-beam axis. Twelve bismuth germinate (BGO) crystals coupled to avalanche photodiodes (APDs) surround the decay region and serve to detect higher energy gammas while three bare APDs directly detect soft x-rays. Having completed its operation on the NG-6 station at the NCNR (NIST Center for Neutron Research) at NIST, Gaithersburg, at the end of 2009, analysis of the RDK II data is nearing completion. We present a summary of the experiment and progress in data analysis and simulations. [Preview Abstract] |
Friday, October 25, 2013 9:54AM - 10:06AM |
HE.00008: Nab: toward a precise experimental characterization of neutron beta decay Dinko Pocanic Nab, a new program of measurements at the Spallation Neutron Source, Oak Ridge, TN, will study unpolarized neutron beta decays, with the goal to determine $a$, the electron--neutrino correlation with precision of $\delta a/a = 10^{-3}$, and $b$, the Fierz interference term, with uncertainty $\delta b \simeq 3\times 10^{-3}$. Neutron beta decay offers a means to study the weak interaction with great precision; its relatively simple theoretical description in the Standard Model (SM) is overconstrained by the set of available observables. Projected Nab results will lead to a new precise determination of the ratio $\lambda=G_A/G_V$, and to significant reductions in the allowed limits for both right- and left-handed scalar and tensor currents. Alternatively, the experiment may detect a discrepancy from SM predictions consistent with certain realizations of supersymmetry. An optimized, asymmetric spectrometer has been designed to achieve the narrow proton momentum response function required to meet the physics goals of the experiment. The apparatus is to be used in a follow-up measurement (abBA) of observables $A$ and $B$ in polarized neutron decay. Nab is fully funded, and is in the construction stage. We discuss the experiment's motivation, expected reach, and method. [Preview Abstract] |
Friday, October 25, 2013 10:06AM - 10:18AM |
HE.00009: Progress Towards a Measurement of the Neutrino Asymmetry in UCN Beta Decay at LANSCE Syed Hasan The UCNB experiment aims at a measurement of the neutrino asymmetry parameter B, the angular correlation between the neutron spin and the anti-neutrino momentum, in the decay of ultracold neutrons, through the detection of electron-proton coincidences. The UCN from the LANL source are polarized via transport through a 7-T field and then transported to a cylindrical decay trap situated within a 1-T superconducting solenoidal spectrometer. In order to measure electron-proton coincidences, the existing UCNA experiment is modified to include a windowless decay trap, and the UCNA detectors are replaced with thick, large area, highly segmented Si detectors. A proton accelerating potential of 20 kV is applied between the decay volume and the Si detectors, to permit the detection of the low-energy recoil protons. This talk will present results from tests of the accelerating potential system in the 1-T spectrometer magnetic field environment, and will report on the status of progress towards the detection of electron-proton coincidences from neutron decay in the spectrometer. [Preview Abstract] |
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