Bulletin of the American Physical Society
2007 APS April Meeting
Volume 52, Number 3
Saturday–Tuesday, April 14–17, 2007; Jacksonville, Florida
Session B16: Electroweak Interactions |
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Sponsoring Units: DNP Chair: Pieter Mumm, University of Maryland Room: Hyatt Regency Jacksonville Riverfront City Terrace 12 |
Saturday, April 14, 2007 10:45AM - 10:57AM |
B16.00001: Precise branching ratios from $\beta -\gamma $ coincidences: the case of $^{^{34}}Ar$. V.E. Iacob, J.C. Hardy, V. Golovko The experiment reported here aims to extract a precise \textit{ft}-value for the superallowed $0^+\to 0^+ \quad \beta ^+$-decay of $^{^{34}}Ar$. Such measurements are essential in testing the Standard Model via the unitarity of the Cabibbo-Kobayashi-Maskawa matrix. One ingredient in the $ft$-value is the branching ratio and, to be useful, it must be determined with a precision of $\sim $0.1{\%} or better. Since the $\beta ^+$-decay of $^{^{34}}Ar$ populates the ground-state as well as excited states of the $^{3^{34}}Cl$ daughter, to determine the branching ratio, we require $\beta -\gamma $ coincidences as well as $\beta $ singles, both with well determined intensities. Gammas were measured with our precisely calibrated HPGe detector whose absolute efficiency is known to 0.2{\%} for energies between 50 and 1400 keV and to 0.4{\%} from 1400 to 3500 keV. For the $\beta $ detector we also require accurate relative efficiencies. To this end, we have compared the recorded beta spectra -- in singles and in coincidence with individual $\gamma $-ray peaks -- with Monte Carlo calculations performed with the DOSRZNRC program (from the EGS package) [1]. Good agreement was obtained over a wide energy range, allowing us to extract precise branching-ratio results. [1] NRCC Report PIRS-701 and http://www.irs.inms.nrc.ca/inms/irs/EGSnrc/EGSnrc.html [Preview Abstract] |
Saturday, April 14, 2007 10:57AM - 11:09AM |
B16.00002: Parity violation in $\vec np\to d\gamma$ at LANSCE Rob Mahurin The longitudinal asymmetry $A_\gamma$ in photons emitted during radiative neutron-proton capture depends cleanly on the neutral current contribution to the weak nucleon-nucleon interaction. The NPDGamma experiment is an effort to measure this asymmetry with precision $1\times 10^{-8}$, which is 10\% of its range of predicted values. A pulsed beam of polarized slow neutrons is incident on a 16\,L parahydrogen target; capture photons are observed in current mode in a cylindrical array of CsI scintillators. In 2006 the NPDGamma collaboration acquired its first production dataset at the Los Alamos Neutron Science Center. In 730 hours running with 50--55\% neutron polarization, photons from some $2\times 10^{14}$ captures were observed. At present the collaboration is engaged in analysis of these data and in relocation of the apparatus to the Spallation Neutron Source, where the increased neutron flux will permit measurement of $A_\gamma$ to the design precision. I will describe the 2006 run and report on the status of the current efforts. [Preview Abstract] |
Saturday, April 14, 2007 11:09AM - 11:21AM |
B16.00003: A High-Efficiency Resonant RF Spin Flipper with Broad Phase Space Acceptance for Pulsed Polarized Cold Neutron Beams Pil-Neyo Seo The NPDGamma collaboration has developed a RF resonant spin flipper to reverse the neutron polarization in a 10 cm$\times $10 cm pulsed cold neutron beam from a $m$=3 supermirror neutron guide with high efficiency over a broad cold neutron energy range. The resonance spin flipper is a RF solenoid directed along the neutron beam and operated in a homogeneous 10-G magnetic field transverse to the neutron beam at Los Alamos Neutron Science Center. An aluminum shield confines the RF field and effectively attenuates electro-magnetic coupling of the RF power to devices outside the rotator. We report the result of spin-flip efficiency measurements where the neutron beam was both polarized and analyzed by optically-polarized $^{3}$He neutron spin filters. The effect of the spin reversal by the rotator on the neutron beam phase space is compared qualitatively to RF neutron spin flippers based on adiabatic fast passage. [Preview Abstract] |
Saturday, April 14, 2007 11:21AM - 11:33AM |
B16.00004: First neutron beta-decay results from the UCNA experiment Brad Plaster The UCNA Collaboration will extract a precise value (goal of 0.2\%) for the neutron beta-asymmetry from measurements of the angular correlation between the neutron spin and the direction of emission of the decay electron in polarized ultracold neutron decay. Ultracold neutrons polarized via transport through a 7.0-Tesla field are directed into the center of our electron spectrometer, consisting of a 10-cm diameter, 3-m long decay trap situated within a highly-uniform 1.0-Tesla solenoidal field. The spiraling decay electrons are detected at both ends of the spectrometer in identical multi-wire proportional chamber and scintillator detector arrays. First results from neutron beta-decay commissioning runs conducted during late-2006, along with results from calibration studies of the electron spectrometer, will be presented. [Preview Abstract] |
Saturday, April 14, 2007 11:33AM - 11:45AM |
B16.00005: Measurement of UCN Spectra and Al Window Fermi Potentials using the Retarding Potential of a Solenoidal Magnet as an Analyzer A.T. Holley One of the 7T polarizer magnets which forms part of the UCNA experiment at LANSCE was used to measure the velocity spectrum of neutrons produced by our ultra-cold neutron (UCN) source. By observing the change in UCN flux through the magnet as the polarizing field was increased, we were able to obtain directly a longitudinal velocity profile for the UCN. Monte Carlo simulations then yield an estimate for the full velocity spectrum. This technique also allows us to evaluate the Fermi potential of foils placed in the experimental geometry. We present results for measurements, performed during the 2006 run period at LANSCE, of the UCN spectrum and two Al foils, one in the high-field region of the analyzer magnet and another covering our $^{3}$He UCN detector. [Preview Abstract] |
Saturday, April 14, 2007 11:45AM - 11:57AM |
B16.00006: Diamond-Like-Carbon Guides for use in Polarized Ultracold Neutron Transport: Production Improvements and Status Russell Mammei The use of ultracold neutrons (UCN) to study fundamental parameters such as angular correlations in polarized beta decay and the neutron's electric dipole moment is well underway. Many of these experiments require non-magnetic/non-conductive UCN guides with high critical velocity, high specularity, and minimal depolarization. Experiments, performed at the Institut Laue-Langevin, have shown that pulsed laser deposited diamond-like-carbon (DLC) on the inside of quartz tubes is an exemplary guide technology to satisfy these requirements. Recent improvements to the production process, including an expanded cleaning procedure, a target rastering system, and the installation of monitoring diagnostics, have resulted in enhanced DLC guide quality. In addition to the guide production developments, a dedicated guide test system has been built to facilitate UCN transmission and lifetime studies. Using the Los Alamos National Lab UCN source, transmission and lifetime measurements of UCN guides fabricated with these improvements were made and will be presented. [Preview Abstract] |
Saturday, April 14, 2007 11:57AM - 12:09PM |
B16.00007: Analysis of Systematic Errors in the MuCap Experiment Thomas Banks The MuCap experiment seeks to measure the rate of ordinary muon capture by the proton to 1\% precision, thereby determining the nucleon's weak induced pseudoscalar form factor, $g_P$, to 7\%. To achieve this goal, we must confront a multitude of systematic effects. We recently completed analysis of our 2004 data set and obtained a capture rate measurement precise to 2.5\%; analysis of data recorded in 2005--2006 is currently under way and should ultimately yield results within the design goal. In this talk I will discuss the methods used to study, quantify, and reduce the systematic errors in the analysis of the 2004 data, and I will examine anticipated improvements for future analysis. [Preview Abstract] |
Saturday, April 14, 2007 12:09PM - 12:21PM |
B16.00008: Precision measurements of muon capture on the proton and deuteron with MuCap Steven Clayton The MuCap collaboration recently released a first result of a measurement of the singlet capture rate $\Lambda_s$ for the muon capture process $\mu+p \rightarrow n+v$, unambiguously determining the pseudoscalar form factor $g_P$ of the charged electroweak current of the proton; The final results of this analysis on $g_P$ and the experimental situation will be presented. As a follow up experiment the collaboration is planning a measurement of the muon capture rate on the deuteron to 1\% precision. This would provide the most accurate experimental information on the axial current interacting with the two nucleon system and determine the low energy constant $L_{1A}$ relevant for solar neutrino reactions and the SNO experiment. [Preview Abstract] |
Saturday, April 14, 2007 12:21PM - 12:33PM |
B16.00009: Discrimination of Rare $\pi^+$ and $\mu^+$ Decays at Rest Using Artificial Neural Networks Emil Frle\v{z} PEN, a new experiment aiming to measure the $\pi^+\to e^+\nu$ decay branching ratio ($\pi_{e2}$) with a relative uncertainty of $\sim 5\cdot 10^{-4}$ is being mounted at the Paul Scherrer Institute ring accelerator. The PIBETA detector was upgraded with new active degrader and fast active target detectors. Low-momentum $\pi^+$ beam tunes in the range 72-80\,MeV/c were developed producing stopping rates of up to 20k\,$\pi^+$/s. Beam detector PMT waveforms were recorded with a 2\,GHz/10 bit Acqiris digitizer. The ``system waveform functions'' for the stopping $\pi^+$'s and $\mu^+$'s and decaying $e^+$'s were determined from a set of clean Michel events. The $\pi^+\to e^+\nu$ events were distinguished from the copious Michel background using artificial neural network analysis. [Preview Abstract] |
Saturday, April 14, 2007 12:33PM - 12:45PM |
B16.00010: Q$_{weak}$: A Precision Measurement of the Proton's Weak Charge Juliette Mammei The weak charge of the proton is a Standard Model-suppressed observable: $Q_{weak}^p \sim 1 - 4\sin^2\theta_w \approx 0.05$. The Q$_{weak}$ experiment proposes to measure this quantity to 4\%. A measurement with this precision is sensitive to new physics at the TeV scale, such as lepto-quarks, $Z'$s, or R-parity violating SUSY. The parity violating asymmetry in elastic electron proton scattering arises from the interference between photon and $Z$ boson exhange, and at low momentum transfers is dominated by $Q_{weak}^p$. The experiment will utilize an 80\% polarized, 180 $\mu$A, 1.165 GeV electron beam scattered from a 35 cm liquid hydrogen target for a production run time of 2200 hours. A resistive toroidal magnet with 8 fold symmetry around the beamline will focus elastically scattered electrons of 8 $\pm$ 2$^\circ$ onto a set of quartz bars. The $Q^2$ corresponding to these kinematics is 0.03 (GeV/c)$^2$, and the expected asymmetry is $A\sim$ -0.23 ppm. At these kinematics the contributions from nucleon structure are suppressed. Due to the high counting rate of $\sim$800 MHz/octant, the experiment will be run in integrating mode. The $Q^2$ distribution will be measured in a low rate counting mode with a set of tracking detectors. An overview of the physics and experiment as well as a status report will be given. [Preview Abstract] |
Saturday, April 14, 2007 12:45PM - 12:57PM |
B16.00011: G0 Backward Angle Measurement: Strange Quark Contributions to Parity-Violating Asymmetries in Electron Scattering from LH2 and LD2 at $Q^2 \sim 0.63$ and $0.23$ GeV$^2$ Mathew Muether The G0 backward angle experiment at Jefferson Lab completed running in March 2007. The experiment measured parity-violating asymmetries from backward scattered electrons in elastic electron-proton and quasi-elastic electron-deuteron interactions at momentum transfers of $Q^2=0.63$ and $0.23$ GeV$^2$. These asymmetries, arising from the interference of the electromagnetic and neutral weak interactions, are expected to be only a few tens of parts-per-million. The final asymmetries together with previous results, e.g. the G0 forward angle measurement [1], will allow for the determination of the strange quark contribution to the proton electric ($G_E^s$) and magnetic ($G_M^s$) form factors as well as the axial form factor ($G_A^e$) at the measured momentum transfers. The status of the experiment and the ongoing analysis of the data from both hydrogen and deuterium targets at beam energies of $687$ and $362$ MeV will be discussed. \newline \newline [1] D.S. Armstrong et al. (G0), {\it Phys. Rev. Lett.} {\bf 95}, 092001 (2005). [Preview Abstract] |
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