Bulletin of the American Physical Society
APS April Meeting 2010
Volume 55, Number 1
Saturday–Tuesday, February 13–16, 2010; Washington, DC
Session H8: Electroweak Interactions II |
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Sponsoring Units: DNP Chair: Geoff Greene, University of Tennessee Room: Delaware B |
Sunday, February 14, 2010 10:45AM - 10:57AM |
H8.00001: Global Analysis of the Strange Vector and Axial Form Factors of the Nucleon and Their Uncertainties John Schaub, Stephen Pate We have studied the strange-quark contributions to the elastic vector and axial form factors of the nucleon, using all available elastic electroweak scattering data sensitive to these contributions. Specifically, we combine elastic $\nu p$ and $\bar{\nu} p$ scattering cross section data from the Brookhaven E734 experiment with elastic $ep$ and quasi-elastic $ed$ and $e-^4$He scattering parity-violating asymmetry data from the SAMPLE, HAPPEx, G0 and PVA4 experiments. We have not only determined these form factors at individual values of momentum-transfer ($Q^2$), as has been done recently, but also have fit the $Q^2$-dependence of these form factors using simple functional forms. We present the results of these fits using existing data, along with some expectations of how our knowledge of these form factors can be improved if a variety of approved and proposed experiments are completed. [Preview Abstract] |
Sunday, February 14, 2010 10:57AM - 11:09AM |
H8.00002: Experimental Constraints on the Parameters of the Effective Field Theory of the Weak Nucleon-Nucleon Interaction William Snow A weak interaction between nucleons is induced by the quark-quark weak interaction in the Standard Model. A description of the weak NN interaction and calculations of parity-odd observables in few nucleon systems using effective field theory (EFT) techniques is under development. I will summarize the present constraints on the weak NN EFT parameters and the prospects for further progress in experiment. [Preview Abstract] |
Sunday, February 14, 2010 11:09AM - 11:21AM |
H8.00003: Measurement of the Parity-odd Neutron Spin Rotation in Liquid Helium H. Erik Swanson, C.D. Bass, T.D. Bass, B.E. Crawford, J.M. Dawkins, K. Gan, B.R. Heckel, J.C. Horton, C.R. Huffer, D. Luo, D.M. Markoff, A.M. Micherdzinska, H.P. Mumm, J.S. Nico, A.K. Opper, M.G. Sarsour, E. Sharapov, W.M. Snow, S.C. Walbridge, V. Zhumabekova A weak interaction between nucleons is induced by the quark- quark weak interaction in the Standard Model. At present the NN weak interaction is poorly constrained by experiment. We conducted an experiment to search for parity violation in the rotation of the plane of polarization of slow neutrons in liquid 4He at the Center for Neutron Research at the National Institute of Standards and Technology. We will discuss the analysis of data from this experiment. [Preview Abstract] |
Sunday, February 14, 2010 11:21AM - 11:33AM |
H8.00004: A Neutron Polarimeter for the Parity Violating Neutron Spin Rotation Measurement in Liquid $^{4}$He D. Luo, C.D. Bass, T.D. Bass, B.E. Crawford, J.M. Dawkins, K. Gan, B.R. Heckel, J.C. Horton, C.R. Huffer, D.M. Markoff, A.M. Micherdzinska, H.P. Mumm, J.S. Nico, A.K. Opper, M.G. Sarsour, E. Sharapov, W.M. Snow, H.E. Swanson, S.C. Walbridge, V. Zhumabekova To better understand the N-N weak interaction, we performed an experiment on the NG-6 beamline at NCNR using polarized cold neutrons passing through a liquid $^{4}$He target to measure the parity violating spin rotation. This is the most sensitive neutron spin rotation experiment performed to date. The polarimeter is designed to isolate the parity-odd component of the rotation, cancel the Larmour precession background from residual magnetic fields to isolate the expected 10$^{-6}$ rad/m parity-odd signal, and reduce nonstatistical noise from neutron source intensity fluctuations. We will discuss the design and operation of the neutron polarimeter. [Preview Abstract] |
Sunday, February 14, 2010 11:33AM - 11:45AM |
H8.00005: Computer Simulation and Systematic Effects for Neutron Spin Rotation in Liquid Helium Kangfei Gan, A.M. Micherdzinska, A.K. Opper, C.D. Bass, H.P. Mumm, J.S. Nico, T.D. Bass, J.M. Dawkins, J.C. Horton, D. Luo, W.M. Snow, B.E. Crawford, B.R. Heckel, H.E. Swanson, C.R. Huffer, P.R. Huffman, D.M. Markoff, M.G. Sarsour, E.I. Sharapov, V. Zhumabekova \newcommand{\np}{\mbox{$\rm n p \rightarrow d\pi^0$}} \newcommand{\pp}{\mbox{$\rm p p \rightarrow d\pi^+$}} \newcommand{\Afb}{\mbox{$A_{fb}$}} \newcommand{\Afbtheta}{\mbox{$A_{fb}(\theta )$}} \newcommand{\etal}{\it et\ al.\rm} \newcommand{\eg}{\it e.g. \rm} \newcommand{\ie}{\it i.e. \rm} A high precision measurement of the parity-violating spin rotation $\phi_{PV}(\vec{n},^{4}He)$ for transversely polarized neutrons passing through liquid $^4$He has taken place at the NIST Center for Neutron Research. To investigate and quantify systematic effects, a computer simulation of the experiment which takes into account neutron optical effects and scattering from liquid helium has been written for neutron transport through the polarimeter and target. Scattering in the 4K helium target is modeled using neutron and light scattering data and theoretical constraints. Limits on systematic effects based on the simulation and measurements have been determined for internal magnetic fields, magnetic field gradients, and possible phase space non-uniformities of the beam and polarization analyzer. [Preview Abstract] |
Sunday, February 14, 2010 11:45AM - 11:57AM |
H8.00006: The status of the NPDGamma experiment and the liquid para-H$_{2}$ target Jiawei Mei The NPD$\gamma$ experiment proposes to measure the asymmetry of the $\gamma$ ray direction relative to the neutron spin when polarized neutrons capture on protons. The asymmetry results from the weak parity-violating hadronic interaction. The goal is to measure the asymmetry with 10$^{-8}$ sensitivity. Such a measurement will provide a theoretically clean value of the weak pion-nucleon coupling, resolving a long standing controversy in nuclear physics. Phase one of the experiment completed at the Los Alamos Neutron Science Center (LANSCE) reports a result consistent with zero: -1.1 $\pm$ 2.1 (stat) $\pm$ 0.2 (sys) $\times \ 10^{-7}$. The experiment is under construction at Oak Ridge National Laboratory, to be installed at the Fundamental Neutron Physics Beam (FNPB) line at the Spallation Neutron Source (SNS). The liquid hydrogen target currently being modified at the Indiana University Cyclotron Facility (IUCF), will be moved to SNS at the end of this year. The phase 2 data will be taken in 2010. I will give an overview describing the status of the experiment and especially the design and performance of the NPD$\gamma$ hydrogen target system. [Preview Abstract] |
Sunday, February 14, 2010 11:57AM - 12:09PM |
H8.00007: Measuring D$_2$O depolarization for NDT$\gamma$ experiment Zhaowen Tang Quark-quark weak interactions in the Standard Model induce nucleon-nucleon weak interactions. Although the NN weak amplitudes are typically at the ppm level compared to NN strong amplitudes, they can be isolated using parity violation. The short range of the quark-quark weak interactions relative to the nucleon size, chiral symmetry breaking in QCD, and quark confinement make theoretical calculations in this sector difficult. However at low energy the number of independent amplitudes is finite and theoretical descriptions using meson exchange model and effective field theory exist. The goal of experimental work in this field is to perform sensitive measurements in few-nucleon systems to determine these amplitudes. A measurement of parity violation in the $\gamma$ asymmetry of polarized n-D capture can determine an interesting linear combination of amplitudes. We will discuss progress toward establishing the feasibility of this experiment and describe a recent measurement at LANSCE of the depolarization of slow neutrons in D$_2$O, which would be used as a target in an eventual experiment. [Preview Abstract] |
Sunday, February 14, 2010 12:09PM - 12:21PM |
H8.00008: High Precision Half-Life Measurement of $^{38}$Ca H.I. Park, J.C. Hardy, V.E. Iacob, L. Chen, J. Goodwin, V. Horvat, N. Nica, L. Trache, R.E. Tribble The measured $ft$ values for superallowed 0$^{+}$ $\rightarrow$ 0$^{+}$ nuclear $\beta$ decay can be used to test the Conserved Vector Current (CVC) hypothesis and the unitarity of the Cabbibo-Kobayashi-Maskawa (CKM) matrix. One of the essential elements of this test is the calculated radiative and isospin-symmetry breaking corrections that must be applied to experimental data [1]. Some of these corrections depend on nuclear structure and their uncertainties can, in principle, be reduced by improving the precision of the experimental $ft$ values. The case of $^{38}$Ca is particularly interesting since its structure-dependent correction is calculated to be one of the largest in the $sd$ shell. The $Q_{EC}$ value of the $^{38}$Ca decay is already well measured [2] and we have now measured its half-life to better than 0.1\% precision. Preliminary results will be presented.\\[4pt] [1] I.S. Towner and J.C. Hardy, Phys. Rev. C {\bf 77}, 025501 (2008).\\[0pt] [2] R. Ringle {\it et al.}, Phys. Rev. C {\bf 75}, 055503 (2007). [Preview Abstract] |
Sunday, February 14, 2010 12:21PM - 12:33PM |
H8.00009: Precision Measurement of the Radiative Decay Mode of the Neutron Benjamin O'Neill The theory of quantum electrodynamics predicts that beta decay of the neutron into a proton, electron, and antineutrino should be accompanied by a continuous spectrum of photons.~ We previously reported detection of photons from neutron beta decay with a branching ratio of (3.13+/- 0.34) $\times $ 10$^{-3}$ in the energy range of 15 keV to 340 keV. This was achieved by prompt coincident detection of an electron and photon, in delayed coincidence with a proton. The photons were detected using a single bar of bismuth germanate scintillating crystal coupled to an avalanche photodiode (APD). Our current experiment employs an array of twelve of these detectors, as well as three large area APD detectors.~ We anticipate that our improved measurement of the branching ratio will have an uncertainty of 1 percent.~ In addition, the large area APDs allow extension of the detectable energy range to $\approx $0.5 keV.~ We will present details of the apparatus and a discussion of the status of the analysis of the branching ratio and the photon energy spectrum. [Preview Abstract] |
Sunday, February 14, 2010 12:33PM - 12:45PM |
H8.00010: Simulations for a Precision Measurement of the Radiative Decay Mode of the Neutron Matthew Bales An experiment is underway to measure the branching ratio (BR) and energy spectrum of the radiative decay mode for the free neutron in the 10 keV to 340 keV range with an anticipated uncertainty of 1{\%} for the BR. The measurement is performed by correlating the detection of the proton, electron, and photon from the beta decay of a cold neutron beam. The beam travels through magnetic and electric fields which transport the charged decay particles to a surface barrier detector located off axis of the beam. Photons are registered by two detectors: an annular array of twelve bismuth germanate crystals coupled to avalanche photodiodes and a separate detector consisting of three bare avalanche photodiodes. The latter will allow extension of the detected photon energy range down to approx. 0.5 keV. Due to the complex nature of the geometry, fields, and materials in the experiment, it is necessary to employ computer simulation to extract the BR. These techniques include event generation, charged particle transport, and detector simulation. By comparing the simulation to measured parameters (such as rates, energy spectra, and time-of-flight), we can verify that the simulations correctly model the physics of the experiment. We present details of the simulation methods and the results as they relate to the data analysis. [Preview Abstract] |
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