Bulletin of the American Physical Society
2012 Fall Meeting of the APS Division of Nuclear Physics
Volume 57, Number 9
Wednesday–Saturday, October 24–27, 2012; Newport Beach, California
Session KE: From Particle Decay to Qweak |
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Chair: Rex Tayloe, Indiana University Room: Garden II |
Friday, October 26, 2012 2:00PM - 2:12PM |
KE.00001: The Nab experiment Christopher Crawford Neutron decay is a clean semi-leptonic process which depends on the vector $G_V$ and axial-vector $G_A$ coupling constants. The ratio $\lambda = G_A/G_V$, which can be extracted from various correlations in decay products of the neutron, is important for determination of $V_{ud}$ in unitarity tests of the CKM matrix, and to test extensions of the standard model. The goal of the newly funded Nab experiment is to measure the electron-neutrino decay correlation $a$ with a relative uncertainty of $10^{-3}$, and the Fierz interference term $b$ with an overall uncertainty of $3\times10^{-3}$. This experiment uses a new technique to determine the electron-neutrino angle from the energy of the electron and proton, detected in coincidence. We will present the physical design and projected sensitivity of this experiment. [Preview Abstract] |
Friday, October 26, 2012 2:12PM - 2:24PM |
KE.00002: New results on muon radiative decay Dinko Pocanic The PIBETA and PEN experiments, a series of precise measurements of rare pion and muon decays at PSI, have acquired a substantial set of $\mu^+ \to e^+ \nu \bar{\nu} \gamma$, radiative muon decay (RMD), events. The measurements were made using a stopped pion beam decaying in an active target, and positron and photon detection in a segmented spherical pure-CsI electromagnetic shower calorimeter covering $\Delta \Omega \simeq 3\pi$ sr, with MWPC central tracking and particle identification. The present RMD study has resulted in approximately 30-fold improvement in the precision of the decay branching ratio for ($E_\gamma > 10$ MeV, and $\theta_{\gamma-e} > 30^\circ$), compared to previous work. Our 1\% result is in excellent agreement with standard model theoretical predictions. Focusing on a narrower range of phase space, we were able to improve significantly the upper limit on the Michel paramter $\bar{\eta}$, which is sensitive to non-(V$-$A) admixtures in the weak lagrangian. [Preview Abstract] |
Friday, October 26, 2012 2:24PM - 2:36PM |
KE.00003: Precise treatment of pion decays in flight in the PEN experiment Emil Frlez The PEN experiment at Paul Scherrer Institute aims to measure the leptonic decay $\pi^+\to e^+\nu(\gamma)$ branching ratio with $5\cdot 10^{-4}$ relative uncertainty. The shower leakage for 70\,MeV monoenergetic positrons in the CsI electromagnetic calorimeter is of the order of 2\,\% for $E_{\rm CALO}<50\,$MeV and presents an important systematic uncertainty. We discuss the ways the low energy $e^+$ tail can be extracted from the measured data and distinguished from $\pi\to\mu\to e$ decays in flight (DIF) that results in a positron in the same $0-50\,$MeV range. We compare the measured $e^+$ energy tail spectra and DIF with realistic GEANT4 simulations done with the photonuclear processes included. [Preview Abstract] |
Friday, October 26, 2012 2:36PM - 2:48PM |
KE.00004: Results from the Measurement of the $^{19}$Ne Half-Life at KVI Leah Broussard We report on the results of the 2009 measurement of the $^{19}$Ne half-life performed at the Trapped Radioactive Isotopes: Microlaboratories for Fundamental Physics (Tri$\mu$p) facility at the Kernfysisch Versneller Instituut (KVI). This system is a member of the set of T=$\frac{1}{2}$ mirror transitions, which can potentially be used to extract $V_{ud}$ with similar precision to the $0^+ \rightarrow 0^+$ decays. A blinded analysis of the data has yielded the half-life $17.2832 \pm 0.0077$ seconds, which is 1.2$\sigma$ from the global average used in a recent review of $\mathcal{F}$t values of the mirror transitions, and 6$\sigma$ from the average of the higher precision results performed at Princeton. We will review the systematic effects which contribute to the uncertainty, and discuss post-unblinding analysis efforts. [Preview Abstract] |
Friday, October 26, 2012 2:48PM - 3:00PM |
KE.00005: Atom trap for 221Fr from 225Ac ion beam implantation M. Tandecki, J.A. Behr, M.R. Pearson, J. Zhang, L. Orozco, R. Collister, G. Gwinner, E. Gomez, S. Aubin A neutral atom trap for francium parity violation experiments is being set up at TRIUMF. The half-lives of the longest isotopes are minutes, which mostly will be produced by the online mass separator of the ISAC facility. For systematic error studies for precision measurements, it can help to have a longer-lived source. $^{221}$Fr is produced by t$_{1/2}$=10 day $^{225}$Ac $\alpha$ decay, and has been trapped at JILA [Z.-T. Lu PRL 79 994 (1997)]. Our approach would implant the mass-separated $^{225}$Ac beam produced by ISAC at $1\times 10^7$/s for a day after the production proton beam is turned off. The scheme to be tested: 30 keV $^{225}$Ac beam is implanted in tantalum for a day; the sample is held in front of an yttrium foil (normally used to stop a mass-separated Fr beam) for 1 minute; 100 keV $^{221}$Fr recoils escape and implant in the yttrium; tantalum is withdrawn, yttrium is moved to trap and heated; cycle repeats. First tests are planned for September, and one goal is precise measurements of atomic hyperfine splittings sensitive to the spatial distribution of nuclear magnetism. [Preview Abstract] |
Friday, October 26, 2012 3:00PM - 3:12PM |
KE.00006: Gamma-ray asymmetry measurements in the capture of polarized cold neutrons on $^{27}$Al Septimiu Balascuta We have measured parity-violating (PV) and parity-allowed (PA) asymmetries in the angular distribution of the gamma rays emitted from the capture of cold polarized neutrons in a solid Al target at the SNS Beam Line 13 at ORNL. The measurement is a part of the systematic studies of the NPDGamma experiment. About 20{\%} of the detector signal in the NPDGamma is produced by neutrons captured in the Al walls and the beam windows of the liquid para-hydrogen target. Therefore it is necessary to measure the strength of PV and PA gamma-ray asymmetries in Al. The sources of instrumental and physical systematic errors in the measured Al data and the contributions of the strong and electromagnetic components of the spin-orbit interaction to the PA asymmetry were identified and will be discussed. [Preview Abstract] |
Friday, October 26, 2012 3:12PM - 3:24PM |
KE.00007: High Precision Polarization Strategy for the Q$_{weak}$ Experiment Juan Carlos Cornejo The Q$_{weak}$ experiment in Hall C at Jefferson Lab is measuring the weak charge of the proton to an uncertainty of 4.1\% by scattering longitudinally polarized electrons from protons in a liquid hydrogen target. The electron beam polarization is budgeted as the largest experimental systematic uncertainty of 1.5\%. Aiming for a polarization measurement to within 1\%, two independent polarimeters monitored the beam polarization by measuring the asymmetry of the polarized electron beam scattering either via the M\o ller or Compton process. The M\o ller polarimeter detects the pair of electrons scattered from a highly polarized thin iron foil through two identical calorimeters. However, it was limited to running at much lower currents and measurements are invasive to the experiment. The Compton polarimeter was commissioned to remove this limitation by using a high powered Fabry-P\'erot cavity laser to provide a polarized photon source. The use of Compton scattering allows for non-invasive measurements and the ability to run at the same currents as the main experiment. The Compton scattered photons and electrons are detected independently. I will discuss the polarimetry strategy and preliminary polarization results for the Q$_{weak}$ experiment using these three techniques. [Preview Abstract] |
Friday, October 26, 2012 3:24PM - 3:36PM |
KE.00008: Compton polarimeter during the Qweak experiment Amrendra Narayan Compton Polarimeter was successfully used during the Q$_{weak}$ experiment from Nov 2010 to May 2012 in Hall C, Jefferson Lab. In this newly installed Compton polarimeter the electron beam collides with green laser stored in a low gain Fabry-Perot Cavity; the scattered electrons are detected in 4 planes of a novel diamond micro strip detector while the back scattered photons are detected in four crystals of PbWO$_4$ detector. The analyzing power is computed using a Geant4 Monte Carlo simulation to match the real detector energy resolution and non-linearity in the response of the detector are folded in. The Compton polarimeter has achieved the design goals of 1\% statistical uncertainty per hour and we expect to achieve 1\% systematic uncertainty. We have done specific tests to understand various properties of the electron and photon detectors. We will share our study of the contributions to the electron and photon detector's systematic uncertainty along with the preliminary polarization results. [Preview Abstract] |
Friday, October 26, 2012 3:36PM - 3:48PM |
KE.00009: Comparison of ``integrating'' and ``tracking'' modes of operation in the Qweak experiment Rob Mahurin The Qweak experiment the first direct measurement of the proton's weak charge $Q_W^p$, has recently completed data collection at Jefferson Lab. Polarized, 1.165~GeV electrons were scattered from protons and focused onto an array of large (2\,m$\times$0.18\,m) fused-silica Cherenkov detectors. We have proposed to measure the parity-violating asymmetry associated with $Q_W^p$ to a precision of 5~ppb. To meet this statistical requirement, the bulk of the data were collected using ``integrating'' electronics (with typical event rates $\sim$800\,MHz per detector). However, the observed asymmetry also depends on the distribution of momentum transfer $Q^2$ accepted by the experiment. Measurements of $Q^2$ were made by inserting wire chamber detectors --- but only with the beam current reduced by three to six orders of magnitude. For these ``tracking'' measurements, the main detectors were connected to electronics able to resolve single events. As a bridge between integrating and tracking modes, a small (1\,cm$\times$1\,cm) Cherenkov detector on a motion stage had freedom to scan over one of the large Cherenkov detectors; the small size permitted use of the same electronics at all beam currents. In this talk I will discuss the consistency of results from these various modes of operation. [Preview Abstract] |
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