Session FE: Mini-Symposium on Fundamental Symmetries with Neutrons I

Initial results from the NPDGamma Experiment at the Spallation Neutron Source

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 (to be measured to $10^{?8}$) from this process can be directly related to the strength of the hadronic weak interaction between nucleons, specifically the $\Delta$I = 1 contribution. As part of the commissioning runs on the Fundamental Neutron Physics beam line at the Spallation Neutron Source at ORNL, the gamma-ray asymmetries from the parity-violating capture of cold neutrons on $^{35}$Cl and $^{27}$Al were measured, to check for systematic effects, false asymmetries, and backgrounds. Early in 2012, the parahydrogen target for the production run of NPDGamma was commissioned. Preliminary results for the commissioning measurements with $^{35}$Cl and $^{27}$Al will be presented. Finally, intermediate results for the hydrogen asymmetry will be shown and discussed.   

n3He: A Measurement of Parity Violation in the Capture of Cold Polarized Neutrons on He-3

The n3He experiment which will run at the SNS aims to measure the parity violating asymmetry in the direction of proton emission from the reaction $\vec{n}(^3He,T)p$ where longitudinally-polarized cold neutrons are captured on unpolarized $^3$He. The neutron spin direction will be reversed at 60 Hz in an 8 step sequence to control systematic effects. The maximum size of the proton asymmetry is calculated to be $3\times 10^{-7}$. The goal of the experiment is to measure this asymmetry with precision of $2\times10^{-8}$ to benchmark the predictions. The $^3$He target serves as a combined target and ion chamber detector. For charge collection, the chamber has 16 signal and 17 HV wire frames, made from macor ceramic. Each signal wire, 9 wires per frame, are read out individually. I will present on the experiment and the target ion chamber.   

Parity-Violating Neutron Spin Rotation in Hydrogen and Deuterium and the Predictive Power of EFT

One of the least-explored sectors of the Standard Model is the weak part of the nuclear force. Experiments on hadronic parity-violation (PV) at low energies require one comprehensive theoretical framework with reliable error-estimates to: check data consistency; subtract binding effects; and extract the PV interaction strengths. ``Pion-less'' Effective Field Theory is such a method with minimal theoretical bias. Different PV parameters are probed in $np$ and $nd$ spin rotation. Using na\"ive dimensional analysis, the signal for standard target densities is $\left|\frac{\mathrm{d} \phi_{\mathrm{PV}}}{\mathrm{d} l}\right|\approx [10^{-7}\cdots10^{-6}]\frac{\mathrm{rad}}{\mathrm{m}} $. An estimate of the numerical and systematic uncertainties of our calculations indicates excellent convergence. We also show that PV 3-nucleon interactions are suppressed in the $nd$-system, despite the non-perturbative renormalisation of parity-conserving 3-nucleon interactions. Therefore, few-nucleon experiments can dis-entangle PV 2-nucleon interactions at the 10\%-level without introducing new unknowns.\\[4pt] [1] H. W. Grie{\ss}hammer, M. R. Schindler and R. P. Springer, Eur. Phys. J. A 48 (2012) 7. \newline [2] H. W. Grie{\ss}hammer and M. R. Schindler, Eur. Phys. J. A 46 (2010) 73   

Physics using Polarized Slow Neutron Spin Rotation

We discuss the physics which can be accessed through measurement of the rotation of the plane of polarization of slow neutrons moving through matter. We isolate the relevant term in the forward scattering amplitude and present examples of phenonema which can in principle give rise to such a rotation, such as neutron-matter weak interactions in the Standard Model [1], exotic weakly-coupled long-range interactions [2], and many forms of gravitational torsion.\\[4pt] [1] W.M. Snow {\it et al.}, Phys. Rev. C {\bf 83} 022501(R) (2011).\\[0pt] [2] H. Yan and W. M. Snow, Phys. Rev. Lett {\bf 110}, 082003 (2013).   

Apparatus Improvements for the Neutron Spin Rotation Device

The NSR collaboration proposes to search for parity violation in n-4He by searching for a rotation of the plane of polarization of transversely polarized neutrons moving through the liquid. This observable is sensitive to a linear combination of isovector and isoscalar weak amplitudes which is orthogonal to those already measured in p-4He. An earlier measurement conducted at NIST reported $ d\theta/dz=[+1.7\pm9.1(\mathrm{stat})\pm1.4(\mathrm{sys.})]\times10^{-7}$~rad/m [1]. We discuss apparatus improvements in preparation for a future experiment with a combined statistical and systematic error of $10^{-7}$~rad/m, which should suffice to see a nonzero effect. Statistical uncertainties and noise will be reduced using the new high fluence NG-C beamline at NIST, a helium reliquefier to eliminate LHe filling down time, and a liquid helium bellows pump for faster oscillation of the parity signal through liquid motion. Systematic errors can be reduced with improvements in magnetic shielding and greater uniformity of polarized neutron optics from new supermirror polarizers and analyzers. \\[4pt] [1] W.M. Snow et al, Phys. Rev. C \textbf{83} 2 2501 (2010).   

High electric field development for the SNS nEDM experiment

The SNS nEDM collaboration is developing an experiment to search for the neutron's electric dipole moment (EDM), using ultracold neutrons (UCNs) stored in superfluid liquid helium, to be run at the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory. In this experiment, being able to achieve a strong and stable electric field in superfluid liquid helium in the region where UCNs are stored is of critical importance, because in EDM searches in general the sensitivity depends linearly on the strength of the applied electric field. However, the phenomenon of electric breakdown in liquid helium is poorly understood, and as such a major R{\&}D effort is under way. The SNS nEDM collaboration has developed an apparatus to study electrical breakdown in liquid helium at temperatures as low as 0.4 K at pressures between the saturated vapor pressure and 1 atm for electrodes 12 cm in diameter with a gap size of a few cm. In this talk, the current status of the high electric field R{\&}D and the implications of the findings on the SNS EDM experiment will be discussed.   

Development of a systematic studies apparatus at NC State for the Neutron Electric Dipole Moment collaboration

An apparatus is being developed for use at the UCN source at the 1 MW NCSU PULSTAR reactor. The goal is to study several critical aspects of the SNS neutron EDM experiment without the electric field. Detailed studies of the interactions between the $^3$He and UCNs, measurements of the correlation functions determining the geometric phase systematic effect, optimization of the parameters for critical dressing, and the pseudomagnetic field caused by neutron scattering from polarized $^3$He will be made. Because of the extremely long turn around times, these would be almost impossible with the SNS apparatus. In this setup, polarized UCNs and $^3$He will be repeatedly loaded into an acrylic cell coated with deuterated TPB and polystyrene on the inner walls. Scintillation light produced in the superfluid $^4$He also in the cell after spin-dependent capture of UCNs on $^3$He will be used to measure the Larmor precession frequency difference between the two species in an external magnetic field. An overview of the experiment will be presented.