Bulletin of the American Physical Society
2006 APS April Meeting
Saturday–Tuesday, April 22–25, 2006; Dallas, TX
Session I9: Minisymposium: Electro-Weak Interactions |
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Sponsoring Units: DNP Chair: Krishna Kumar, University of Massachusetts Room: Hyatt Regency Dallas Cumberland B |
Sunday, April 23, 2006 10:30AM - 11:06AM |
I9.00001: Parity-violating electron scattering on hydrogen and helium and strangeness in the nucleon Invited Speaker: Parity violation in elastic electron scattering is sensitive to possible strange quark contributions to the vector structure of the nucleon, and thus provides an opportunity to isolate effects of the $q\overline{q}$ sea. The small parity-violating asymmetry in the cross section for the scattering of polarized electrons, which arises from $\gamma-Z$ interference, when combined with the known electromagnetic form factors, provides access to strange quark matrix elements. The HAPPEX collaboration in Hall A at Jefferson Lab has measured the parity-violating asymmetry in the scattering of longitudinally-polarized 3 GeV electrons from both hydrogen and $^4$He cryogenic targets, at a small scattering angle ($6^{\circ}$) and low four-momentum transfer ($Q^2$ = 0.1 GeV$^2$). The asymmetry for hydrogen is a function of a linear combination of $G_E^s$ and $G_M^s$, the strange quark contributions to the electric and magnetic form factors of the nucleon respectively, and that for $^4$He is a function solely of $G_E^s$. The combination of the two measurements therefore allows $G_M^s$ and $G_E^s$ to be separately determined. Preliminary results will be presented from the complete data set, obtained in runs in 2004 and 2005, yielding results of unprecedented precision. [Preview Abstract] |
Sunday, April 23, 2006 11:06AM - 11:18AM |
I9.00002: Performance of the beta-spectrometer for a precision measurement of the neutron beta-decay asymmetry B. Plaster, B.W. Filippone, J. Hsiao, T.M. Ito, J.W. Martin, J. Yuan A precise value for the neutron beta-decay asymmetry will be extracted 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 will be directed into the center of our beta-spectrometer, consisting of a 10-cm diameter, 3-m long open-ended decay trap situated within a highly-uniform 1.0 Tesla solenoidal field. Spiraling decay electrons will be detected at both ends of the decay trap in identical detector arrays consisting of a multi- wire proportional chamber backed by plastic scintillator. Initial results of various performance tests of our beta-spectrometer and electron detector system will be presented. [Preview Abstract] |
Sunday, April 23, 2006 11:18AM - 11:30AM |
I9.00003: Observation of the radiative decay mode of the neutron T.R. Gentile, K.J. Coakley, M.S. Dewey, H.P. Mumm, J.S. Nico, A.K. Thompson, B.M. Fisher, I. Kremsky, F.E. Wietfeldt, R.L. Cooper, T.E. Chupp, E.J. Beise, K.G. Kiriluk, J. Byrne We report observation of the radiative decay mode of the neutron, in which the usual decay products are accompanied by an inner-bremsstrahlung photon. The experiment was carried out at the fundamental physics beam line at the National Institute of Standards and Technology (NIST) Center for Neutron Research (NCNR) Photons with energies between 15 keV and 340 keV were detected by a scintillating crystal coupled to an avalanche photodiode and were distinguished from uncorrelated background photons by coincidence with both the decay electron and proton. Correlated background from prompt external bremsstrahlung generated in the electron detector is estimated to be a small effect. We have studied the predicted dependence of the radiative process on the available phase space of the decay, which provides a unique signature of radiative decay. We report our measurements of the branching ratio for this rare decay. [Preview Abstract] |
Sunday, April 23, 2006 11:30AM - 11:42AM |
I9.00004: Monte Carlo Simulation and Photon Detector Development for the Radiative Decay Experiment R.L. Cooper, T.E. Chupp, K.J. Coakley, M.S. Dewey, T.R. Gentile, H.P. Mumm, J.S. Nico, A.K. Thompson, B.M. Fisher, I. Kremsky, F.E. Wietfeldt, E.J. Beise, K.G. Kiriluk, J. Byrne We have recently observed the radiative decay mode of the neutron, where an electron and photon are observed in coincidence, followed by a delayed proton. A false signal can be obtained if bremsstrahlung from the electron detector reaches the photon detector. We estimate the contribution from this process to be small. The potential false signal was experimentally addressed by measuring the dependence of the radiative decay process on the available phase space of decay and comparing it to prediction. Phase space was controlled by varying the voltage on an electrostatic mirror to reflect decay protons. We discuss the simulation techniques used to study the systematic effects in the experiment. We also discuss the performance of the photon detector, which operates in a high magnetic field and at cryogenic temperatures. Additionally we present the design of a 12-element scintillation detector that will allow a precision measurement of the radiative decay spectrum. [Preview Abstract] |
Sunday, April 23, 2006 11:42AM - 11:54AM |
I9.00005: Measurement of the Beta Neutrino Correlation in Laser-Trapped 21Na Paul Vetter, Jamil Abo-Shaeer, Reina Maruyama, Stuart Freedman We have measured the beta neutrino correlation in the decay of radioactive $^{21}$Na atoms confined in a magneto-optic trap using a technique which detects the low-energy electrons shaken off by the daughter recoil atom ($^{21}$Ne). Using a time-of-flight momentum analysis technique, we measure the energy spectrum of the ionized recoil nuclei in coincidence with the shake-off electrons to determine the beta-neutrino correlation. A major source of systematic error in this type of experiment arises from photoassociative binding of the trapped atoms into cold, trapped dimer molecules, despite the small number of trapped atoms. When an atom bound in a molecule beta decays, the momentum of the recoil nucleus is strongly perturbed by scattering from its molecular partner. We have studied the formation rate of cold dimer molecules in our trap, and have implemented a dark trap. This trap suppresses the fraction of atoms which populate the atomic excited state (necessary for photoassociation) and suppresses the molecule formation rate by roughly three orders of magnitude. Current results will be presented. [Preview Abstract] |
Sunday, April 23, 2006 11:54AM - 12:06PM |
I9.00006: Muon Decay Parameters from Majorana Neutrino Masses Rebecca Erwin, Mikhail Gorshteyn, Michael Ramsey-Musolf The TWIST experiment at TRIUMF will probe the Michel parameters, which parameterize contributions to muon decay from beyond the Standard Model, to unprecedented precision. Because some of these parameters are sensitive to operators that also contribute to Majorana neutrino mass, limits on the mass can be used to put constraints on these operator coefficients. In this talk, we will discuss our analysis of these constraints and their subsequent implications for the Michel parameters. [Preview Abstract] |
Sunday, April 23, 2006 12:06PM - 12:18PM |
I9.00007: Precise Branching Ratio Measurement for the $\beta$ Decay of $^{21}$\textbf{Na} V.E. Iacob, J.C. Hardy, C.A. Gagliardi, J. Goodwin, N. Nica, H.-I. Park, G. Tabacaru, L. Trache, R.E. Tribble, Y.J. Zhai, I.S. Towner A recent paper [1] reports a measurement of the $\beta -\nu $ correlation coefficient for the ground-state mirror $\beta ^{+}$ transition $^{21}Na(3/2^+)\to ^{21}Ne(3/2^+)$ of $a_{\beta \nu } =\mbox{0.5243(91)}$, which is significantly different from the Standard Model prediction of 0.558. However, both these values depend on the transition branching ratio, the currently accepted value of which, 94.97(13) {\%}, is an average over mutually inconsistent measurements (25 to 45 years old!). We set out to improve this value. We used a 28$\cdot A$ MeV $^{22}$Ne beam from the Texas A{\&}M cyclotron to initiate the $^{1}$H($^{22}$Ne, 2n)$^{ 21}$Na reaction. The recoils passed through the MARS recoil separator and, after being degraded, the $^{21}$Na ions were implanted as a $>$99{\%} pure source in the tape of a fast transport system. After a few-second collection time, the beam was turned off and the implanted source moved in 175 ms to a shielded region where it stopped between a plastic scintillator and a HPGe detector that is efficiency calibrated with high precision (0.2{\%} between 50 and 1400 keV [2]). Both $\beta $ singles and $\beta -\gamma $ coincidences were then recorded for 60s, and the collect/move/detect cycle was repeated to achieve the desired statistical accuracy. Our result for the ground-state branching ratio is 95.27(5){\%} . [1] N.D. Scielzo \textit{et al.}, Phys. Rev. Lett. \textbf{93 }102501 (2004). [2] R.G. Helmer \textit{et al.}, Nucl. Instr. Meth. \textbf{A511}, 360 (2003). [Preview Abstract] |
Sunday, April 23, 2006 12:18PM - 12:30PM |
I9.00008: Prospects for Testing Standard Model (SM) Extensions with Electron Electric Dipole Moment (EDM) Experiments Harvey Gould, Jason M. Amini, Charles T. Munger Split SUSY and other SM extensions with new sources of CP violation allow electron EDMs that approach the present experimental limits. A new generation of laboratory experiments using laser-cooled atoms is expected to push back those limits, further constraining theory or discovering new physics. A demonstration laser-cooled-atom fountain electron EDM experiment operating at LBNL will be described and the EDM sensitivity and immunity to systematics, of a full scale experiment, discussed. [Preview Abstract] |
Sunday, April 23, 2006 12:30PM - 12:42PM |
I9.00009: Neutrino Mass Implications for Muon Decay Parameters Jennifer Kile, Rebecca Erwin, Michael Ramsey-Musolf, Peng Wang We use the scale of neutrino mass to derive model-independent naturalness constraints on possible contributions to muon decay parameters from new physics above the electroweak symmetry-breaking scale. Focusing on Dirac neutrinos, we obtain a complete basis of dimension four and dimension six operators that are invariant under the gauge symmetry of the Standard Model and that contribute to both muon decay and neutrino mass. We show that--in the absence of fine-tuning--the most stringent bounds on chirality-changing operators relevant to muon decay arise from one-loop contributions to neutrino mass and obtain bounds on the operator coefficients that are approximately 100 times stronger than bounds previously obtained in the literature. We arrive at bounds on the corresponding contributions to the Michel parameters that are six or more orders of magnitude below present experimental limits. [Preview Abstract] |
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