Bulletin of the American Physical Society
APS April Meeting 2011
Volume 56, Number 4
Saturday–Tuesday, April 30–May 3 2011; Anaheim, California
Session B10: Electroweak Interactions |
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Sponsoring Units: DNP Chair: Timothy Chupp, University of Michigan Room: Garden 1 |
Saturday, April 30, 2011 10:45AM - 10:57AM |
B10.00001: The Electric Dipole Form Factor of the Nucleon U. van Kolck, E. Mereghetti, J. de Vries, R.G.E. Timmermans We use chiral perturbation theory in next-to-leading order to calculate the electric dipole form factor of the nucleon at low momentum. We consider effects from sources of time-reversal violation in the Standard Model up to effective dimension six: the QCD theta angle, the light quarks' electric and color-electric dipole moments, and the gluon color-electric dipole moment. [Preview Abstract] |
Saturday, April 30, 2011 10:57AM - 11:09AM |
B10.00002: The NPDGamma experiment - A measurement of parity violation in polarized cold neutron capture on parahydrogen Nadia Fomin The NPDGamma experiment aims to measure the correlation between the neutron spin and the direction of the emitted photon in neutron-proton capture. An up-down parity violating asymmetry from this process can be directly related to the strength of the hadronic weak interaction between nucleons. The methodology and results from the first run of this experiment, completed at LANSCE in 2006, will be discussed. The next phase of the experiment is currently being commissioned on the Fundamental Neutron Physics Beamline of the Spallation Neutron Source at ORNL. We will discuss the improvements in the apparatus and show commissioning data. The upcoming run is expected to yield a measurement with a projected statistical error of 1x10$^{-8}$ as well as negligible systematic errors. This will finally allow the result can be compared with theoretical predictions. [Preview Abstract] |
Saturday, April 30, 2011 11:09AM - 11:21AM |
B10.00003: Limit on parity-violating neutron spin rotation in a liquid 4He target Kangfei Gan The nucleon-nucleon (NN) weak interaction is one of the more poorly understood areas of the Standard Model. Present theoretical models are essentially phenomenological, and experiments in few-nucleon systems do not determine all of the weak NN amplitudes. We have performed the first phase of a high precision measurement of parity-violating (PV) neutron spin rotation. The rotation angle of transversely polarized cold neutrons passing through a liquid $^4$He target was measured to be $d\phi/dz = (+1.7 \pm 9.1 (stat) \pm 1.4(sys)) \times10^{-7}$ rad/m, which is the most precise measurement of PV spin rotation to date. The uncertainty is smaller than current estimates of the range of possible values of $d\phi/dz$ in the n-$^4$He system. The result will be presented along with a discussion of the theoretical motivation, experimental design, analysis for data and systematic effects, and plans for the next phase of the experiment. [Preview Abstract] |
Saturday, April 30, 2011 11:21AM - 11:33AM |
B10.00004: Precise Lifetime Measurement of $^{37}$K P.D. Shidling, R.S. Behling, J.C. Hardy, V.E. Iacob, M. Mehlman, D. Melconian, B.T. Roeder, H. Stephens To determine the correlation parameters of the $\beta $ decay of transitions between isobaric analogue states, the \textit{ft} value is needed to determine $\rho $, the ratio of Gamow-Teller to Fermi matrix elements. A recent review of all $T = \frac{1}{2} \beta$ mirror decays [1] indicates that $^{37}$K is one of the best candidates for testing the Standard Model. The \textit{ft} value is currently limited by the 0.6{\%} uncertainty in the lifetime. In order to make Standard Model predictions of the correlation parameters negligible compared to planned experiments, we have performed a precision lifetime measurement of $^{37}$K. We used the MARS separator for producing a secondary beam of $^{37}$K with a purity of 98.5{\%}. We implanted the activity in an aluminized-Mylar tape that, as part of a fast-tape drive system, quickly transported the sample to a well shielded location, stopping it in the centre of a 4$\pi $ proportional gas counter. The recorded data was separated into 25 runs, each characterized by a different combination of detector high voltages, discriminator thresholds and dominant dead-time settings. An overview of the experiment and results will be presented. \\[4pt] [1] O. Naviliat-Cuncic and N. Severijns, Phys. Rev. Lett. \textbf{102}, 142302 (2009) [Preview Abstract] |
Saturday, April 30, 2011 11:33AM - 11:45AM |
B10.00005: A Study of the Superallowed $\beta^{+}$ Decay of $^{38}$Ca H.I. Park, J.C. Hardy, V.E. Iacob, L. Chen, J. Goodwin, V. Hovart, N. Nica, L. Trache, R.E. Tribble To test the unitarity of the CKM matrix via precise measurements of superallowed $0^{+}\rightarrow 0^{+}$ nuclear $\beta^{+}$ decay, accurate calculations of small nuclear- structure-dependent corrections are essential. Currently, uncertainties in these calculations are comparable to the present level of experimental precision. However, these theoretical uncertainties can, in principle, be reduced by experiment. $^{38}$Ca is a particularly favorable case for this purpose because its nuclear-structure-dependent correction term is calculated to be one of the largest in the $sd$ shell [1]. We report our measured half-life to be 443.88(36) ms and the first preliminary results from a branching-ratio measurement. Combined with the well-known $Q_{EC}$ value [1,2], these two results will ultimately lead to a precise ${\cal F}t$ value for $^{38}$Ca, and a valuable test of the calculated nuclear- structure-dependent correction term.\\[4pt] [1] J.C. Hardy and I.S. Towner, Phys. Rev. C 79, 055502 (2009).\\[0pt] [2] T. Eronen $\textit{et al.}$, to be published. [Preview Abstract] |
Saturday, April 30, 2011 11:45AM - 11:57AM |
B10.00006: Calibrating the Photon Detector for an Experiment Measuring Radiative Decay of the Neutron Robert Cooper Data acquisition has been completed on an experiment to measure the radiative decay mode of the neutron by registering the correlated proton, electron, and photon decay products. This experiment anticipates measuring the branching ratio and the photon energy spectrum to an uncertainty of 1\%. To achieve this precision, the photon detector solid angle coverage was increased by a factor of 12. The detector consists of 12 bismuth germanate scintillating crystals coupled to avalanche photodiodes, and it operates in the cryogenic, high magnetic field environment of a superconducting solenoid. In the initial experiment, the dominant systematic uncertainty was photon detector resolution and gain drift, and to reduce this uncertainty a detailed calibration program was implemented. The background gamma ray spectrum was periodically measured to monitor for possible gain shifts during the run. Radioisotope calibrations were also performed to quantify the detector gain, efficiency, spatial dependence, and resolution. We present the details of these calibrations and describe the simulations used to interpret these data. [Preview Abstract] |
Saturday, April 30, 2011 11:57AM - 12:09PM |
B10.00007: Electron energy response of the NIST RDKII spectrometer and studies of the Fierz interference Matthew Bales In the beta-decay of neutrons and nuclei, non V-A interactions may introduce an electron energy dependence into the decay rate through the Fierz coefficient b, characterizing interference of S-V and/or T-A amplitudes. Experimental limits have been put on the b-coefficient in nuclear systems but not yet in free neutron decay. This has been due to difficulties both in producing free neutrons in a low background environment and in isolating the energy dependence of any instrumental response. We have undertaken an analysis of the electron energy spectrum using data from a radiative neutron decay experiment, RDKII, which utilized a cold neutron beam-line at NIST, Gaithersburg, Maryland. In the experiment, charged particles from neutron decay are transported towards a silicon surface barrier detector using magnetic and electric fields. We have employed direct calibration with electron sources and Monte Carlo simulations such as Geant4 and MCNP5 in order to understand the detector's response function and to evaluate the effects of charged particle transport on the spectra observed. We present our current methodologies, progress, and potential capabilities in determining a Fierz interference term for free neutron decay. [Preview Abstract] |
Saturday, April 30, 2011 12:09PM - 12:21PM |
B10.00008: Improved Measurement of the $pp\mu$ Molecular Formation Rate Sara Knaack The MuCap experiment measures the $\mu^{-}$ lifetime in a gaseous hydrogen TPC to obtain the $\mu$p singlet capture rate $\Lambda_{S}$ by comparing it to the free $\mu^{+}$ lifetime. The motivation is a determination of the pseudoscalar coupling $g_p$ of the proton, which relates linearly to $\Lambda_{S}$. A $\mu^{-}$ atom, formed after a muon stops in the TPC, can form molecular hydrogen at the rate $\lambda_{pp\mu}$, a process that competes with muon capture. Muon capture from the molecular state is slower than from the singlet state and $\lambda_{pp\mu}$ must be known to correctly extract $\Lambda_{S}$ from the data. New results are reported on $\lambda_{pp\mu}$ based on data obtained using an argon-doped hydrogen gas, but under otherwise identical TPC conditions as the un-doped hydrogen data. Argon introduces additional atomic processes involving the muon, which are observed in the muon decay electron and the direct capture neutron time spectra. Fits to these data determine $\lambda_{pp\mu}$ to a precision that is a significant improvement to the current world average. New MuCap data will reduce $\delta\Lambda_{S}$ by a factor of two or more, making the improved measurement of $\lambda_{pp\mu}$ critical. [Preview Abstract] |
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