Bulletin of the American Physical Society
2011 Fall Meeting of the APS Division of Nuclear Physics
Volume 56, Number 12
Wednesday–Saturday, October 26–29, 2011; East Lansing, Michigan
Session NC: Electroweak Interactions III |
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Chair: Chris Crawford, University of Kentucky Room: 101 |
Saturday, October 29, 2011 10:30AM - 10:42AM |
NC.00001: First Calculation of Nuclear Parity Violation from Lattice QCD Joseph Wasem While parity violating phenomena have been known and observed for several decades little is known about the flavor-conserving, parity violating interaction between quarks. The primary example of this interaction is the small parity violating interaction between nucleons. The large QCD background for this interaction makes both experimental extractions and theoretical predictions difficult. Here we report on results from a lattice QCD calculation of $h_{\pi{NN}}$, the leading-order momentum-independent parity violating coupling between pions and nucleons. This is the first calculation of this quantity directly from QCD. [Preview Abstract] |
Saturday, October 29, 2011 10:42AM - 10:54AM |
NC.00002: Hadronic parity violation in pionless effective field theory Matthias Schindler The weak interaction between quarks induces a parity-violating component in the low-energy interaction between nucleons. Due to the nonperturbative nature of QCD at these energies, an understanding of how the weak quark-quark interactions manifest themselves in nucleon interactions remains elusive. Few-nucleon experiments utilizing polarized neutrons are being performed at the SNS, NIST, and other neutron facilities to map out this weak component of the nuclear force. I will describe a theoretical program to analyze and interpret the obtained data based on effective field theory. This approach allows for a systematic and model-independent description of few-nucleon observables. Results for parity-violating observables in the two- and three-nucleon sectors will be presented, including a discussion of the relevance of parity-violating three-nucleon interactions. Recent progress in the application of effective field theory methods in few-nucleon systems will allow us to extend these calculations to observables involving four and more nucleons. [Preview Abstract] |
Saturday, October 29, 2011 10:54AM - 11:06AM |
NC.00003: First Results from the NPDGamma Experiment at the Spallation Neutron Source Nadia Fomin The NPDGamma experiment aims to measure the parity-odd correlation between the neutron spin and the direction of the emitted photon in neutron-proton capture. A 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 summarized. The next phase of the experiment has finished a very successful commissioning on the Fundamental Neutron Physics Beamline of the Spallation Neutron Source at ORNL. We will discuss the improvements in the apparatus and show results from the 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 result will finally test the theoretical predictions. [Preview Abstract] |
Saturday, October 29, 2011 11:06AM - 11:18AM |
NC.00004: Parity-violtating asymmetry from the capture of polarized neutrons on 27AL Septimiu Balascuta, Ricardo Alarcon, James D. Bowman, Seppo Penttila The NPDGamma experiment at the Fundamental Neutron Physics Beamline at the Spallation Neutron Source seeks to determine the strength of the hadronic weak interaction by measuring the parity-violating asymmetry in the angular distribution of the gamma rays emitted in the capture of polarized cold neutrons on protons. The photons are detected in an array of 48 CsI detectors cylindrically arranged around a liquid hydrogen target and about 15\% of the measured photon yield is from the Al target vessel. It is therefore necessary to independently measure the parity-violating asymmetry from Al to quantify it and then correct the measured asymmetry obtained from the hydrogen target accordingly. We have measured the parity- violating photon asymmetry from the capture of polarized cold neutrons on an Al target using the SNS pulsed cold neutron beam and the NPDGamma experimental setup. The results are compared with a theoretical calculation of the capture of polarized cold neutrons in the Al vessel of the hydrogen target done by simulating the gamma cascade emitted from the excited ${}^{28} $Al nucleus and of the bremsstrahlung radiation of the electrons emitted in the nuclear beta decay of ${}^{28}$Al. [Preview Abstract] |
Saturday, October 29, 2011 11:18AM - 11:30AM |
NC.00005: A Liquid parahydrogen target for the NPDGamma experiment Zhaowen Tang The goal of NPDGamma experiment is to measure the parity-odd directional gamma-ray asymmetry from the polarized neutron-proton capture reaction. This reaction is sensitive to the $\Delta I =1$ part of the hadronic weak interaction between nucleons, where contributions from quark-quark neutral currents are expected to dominate. Our goal is to measure $A_\gamma$ to a sensitivity of $10^{-8}$ at the newly commissioned fundamental neutron physics beamline (FnPB) at the Spallation Neutron Source at Oak Ridge National Lab. The target has to meet strict requirements set by this sensitive measurement. It is operated in the para-hydrogen state at 17K to avoid depolarization of cold neutrons. The design of the target has to meet SNS hydrogen safety requirements and minimize background from neutron capture in aluminum. The previous version of the target was successfully operated at LANSCE, where a parahydrogen fraction of 99.98\% was measured [1]. We will describe the target along with its installation and testing. \\[4pt] [1] S. Santra et al, Nucl. Instrum. Methods, A 620, 421-436 (2010) [Preview Abstract] |
Saturday, October 29, 2011 11:30AM - 11:42AM |
NC.00006: GEANT4 Simulation of Empty Target for the NPDGamma Experiment Andrew McNamara The NPDGamma experiment was designed to detect a very small parity violating asymmetry ($\sim 10^-8$) in the n + p $\to$ d + $\gamma$ reaction. Background rates will be modeled to determine the dilution of the asymmetry. To validate these models, data were taken with the detector array fitted with a mock-up version of the liquid hydrogen target in the neutron beam. Rates were measured from four targets: empty, water, and two concentrations of MgCl, which has a large parity violating asymmetry. A comparison of these results with a Geant4 simulation of this simple target will be used to tune the simulation of the real hydrogen target. [Preview Abstract] |
Saturday, October 29, 2011 11:42AM - 11:54AM |
NC.00007: Determining the Neutron Polarization of the Fundamental Neutron Physics Beamline at the Spallation Neutron Source with a $^{3}$He Spin Filter Matthew Musgrave The Fundamental Neutron Physics Beamline (FNPB) at the Spallation Neutron Source (SNS) provides a pulsed beam of polarized cold neutrons for several experiments including NPDGamma and n3He. The neutrons are polarized by a multi-channel super mirror polarizer. The polarization of the neutron beam was measured at several points across the beam cross section by utilizing the spin dependent capture cross section of the neutron on polarized $^{3}$He. The neutron beam is incident on a series of $^{6}$Li loaded collimators and a $^{3}$He spin filter. The transmitted neutrons are detected in a $^{3}$He monitor operating in current mode. An RF spin rotator reverses the spin of the neutrons on successive accelerator pulses enabling the transmission of different neutron spin states to be measured, and the $^{3}$He spin is reversed by adiabatic fast passage to enable the transmission through different $^{3}$He spin states. The neutron polarization can be determined in a redundant fashion using these two independent spin reversals. [Preview Abstract] |
Saturday, October 29, 2011 11:54AM - 12:06PM |
NC.00008: $^3$He multi-wire proportional counters for the FNPB at the SNS Mark McCrea We have constructed a set of beam monitors for the Fundamental Neutron Physics Beam Line (FNPB), at the Spallation Neutron Source, Oak Ridge National Laboratory. The beam monitors are $^3$He multi-wire proportional counters. A $^3$He nucleus that captures a neutron will break up by the reaction $n+^3He\rightarrow p+T+765keV$. The 765keV is released as kinetic energy of the proton and triton which will ionize the chamber gas, giving a consistent signal from each capture. The chamber gas is a mix of $N_2$ and $^3He$ at 750 Torr. The $^3He$ fraction used determines the fraction of the beam that is captured. The $^3$He chambers are used to monitor the neutron flux along the neutron beam, and are currently used for the NPDGamma experiment, but will also be used for beam line diagnostics in future experiments. I will report on the monitor design, construction, and beam data obtained during the commissioning of the NPDGamma experiment. I will also report on the design, and simulation of a $^3$He wire chamber to be used in the n$^3$He experiment, which runs after the NPDGamma experiment. It uses the same neutron detection process as described above, but will be black to neutrons (high $^3He$ content) with a small amount of ionization gas, to allow the protons to range out over as long a distance as possible to measure the parity violating longitudinal asymmetry in the number of protons emitted in the capture reaction. [Preview Abstract] |
Saturday, October 29, 2011 12:06PM - 12:18PM |
NC.00009: Parity-Violating Gamma-ray Asymmetry in Polarized Neutron Capture on $^{35}$Cl Elise Martin As part of the commissioning of the NPDGamma experiment at the Fundamental Neutron Beamline at the Spallation Neutron Source at Oak Ridge National Lab, we measured the gamma-ray asymmetry from the parity-violating interaction of polarized cold neutron capture on chlorine. Previous measurements result in a chlorine asymmetry on the order of 10$^{-5}$, with a precision of 10$^{-6}$. We expect to improve the precision of this value. For the NPDGamma experiment we will measure the gamma-ray asymmetry of hydrogen. Since the asymmetry of chlorine is two orders of magnitude larger than the predicted hydrogen asymmetry, we gain insight into the systematic effects and statistical error introduced by our experimental apparatus. The chlorine measurement, data, and preliminary results will be presented. [Preview Abstract] |
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