Session CG: Mini-Symposium on Electro-weak Physics and Fundamental Symmetries I

Overview: Parity Violation and Fundamental Symmetries

The fields of nuclear and particle physics have undertaken extensive programs of research to search for evidence of new phenomena via the precision measurement of observables that are well predicted within the standard model of electroweak interaction. It is already known that the standard model is incomplete as it does not include gravity and dark matter/energy and therefore likely the low energy approximation of a more complex theory. This talk will be an overview of the motivation, experimental methods and status of some of these efforts (past and future) related to precision in-direct searches that are complementary to the direct searches underway at the Large Hadron Collider.   

Search for the permanent electric dipole moment of $^{129}$Xe

CP-violation in Beyond-the-Standard-Model physics, necessary to explain the baryon asymmetry, gives rise to permanent electric dipole moments (EDMs). EDM measurements of the neutron, electron, paramagnetic and diamagnetic atoms constrain CP-violating parameters. The current limit for the $^{129}$Xe EDM is $6\times10^{-27}~e\cdot\mathrm{cm}$ (95$\%$ CL). The HeXeEDM experiment at FRM-II (Munich Research Reactor) and BMSR-2 (Berlin Magnetically Shielded Room) uses a stable magnetic field in a magnetically shielded room and $^3$He comagnetometer with potential to improve the limit by two orders of magnitude. Polarized $^3$He and $^{129}$Xe free precession is detected with SQUID magnetometers in the presence of applied electric and magnetic fields. Conclusions from recent measurements will be presented.   

Los Alamos nEDM Experiment and Demonstration of Ramsey’s Method on Stored UCNs at the LANL UCN Source

The Los Alamos National Laboratory ultracold neutron (UCN) source was recently upgraded for a factor of 5 improvement in stored density, providing the statistical precision needed for a room temperature neutron electric dipole moment measurement with sensitivity $\approx 3\times 10^{-27} e\cdot$cm, a factor 10 better than the limit set by the Sussex-RAL-ILL experiment. Here, we show results of a demonstration of Ramsey's separated oscillatory fields method on stored UCNs at the LANL UCN source and in a geometry relevant for a nEDM measurement. We argue a world-leading nEDM experiment could be performed at LANL with existing technology and a short lead time, providing a physics result with sensitivity intermediate between the current limit set by Sussex-RAL-ILL, and the anticipated limit from the complex, cryogenic nEDM experiment planned for the next decade at the ORNL Spallation Neutron Source (SNS-nEDM).   

A Search for Possible Exotic Spin Dependent Interactions Using Neutron Spin Rotation

Various theories beyond the Standard Model predict the existence of new particles with masses in the sub-eV range with very weak couplings to ordinary matter. A parity-even and time-reversal-even interaction between polarized nucleons and unpolarized matter proportional to ${g_A}^2\vec{\sigma}\cdot{(\vec{p}\times \vec{r})}$ is one such possibility, where $\vec{\sigma}$ and $\vec{p}$ are the spin and the momentum of the polarized nucleon, $\vec{r}$ is the separation between the polarized nucleon and a nucleon in unpolarized matter, and $g_A$ is the axial vector coupling of an interaction induced by the exchange of a new light vector boson. I present recent results on behalf of the NSR (Neutron Spin Rotation) Collaboration from an experiment performed at the FP12 slow neutron beam line at LANSCE which utilizes a polarized neutron beam to test for the above mentioned possible spin-coupled force. By sending a beam of polarized cold neutrons near thin, flat sheets of non-magnetic, unpolarized matter which could induce a net rotation of the plane of polarization of the neutrons, we can place a limit on the strength of the new possible interaction, $g_A^2$, by measuring the change in neutron polarization downstream of the target.   

Non-Resonant Inelastic $\vec{e}p$ Scattering as a Probe of Hadron Structure

The $Q_{weak}$ experiment at Jefferson Lab made the first direct measurement of the proton's weak charge, $Q^p_W$, via a measurement of the parity-violating asymmetry in elastic $\vec{e}p$ scattering with low four-momentum transfer. Energy-dependent electroweak radiative corrections must be applied to the measured asymmetry. The $\gamma Z$ box, where a photon and a Z-boson are simultaneously exchanged, is the most problematic correction. It cannot be calculated through the same perturbative methods as the rest of the corrections. The asymmetry arising from the $\gamma Z$ box depends on the $\gamma Z$ interference structure functions, $F_{1,3}^{\gamma Z}$, for which there is almost no data. Using the $Q_{weak}$ apparatus, with modifications, a measurement of the parity-violating asymmetry of non-resonant inelastic $\vec{e}p$ scattering was made with \SI{3.35}{GeV} incident electrons. The asymmetry of inelastically scattered electrons with $Q^2 \approx \SI{0.09}{(GeV/c)^2}$ and $W \approx \SI{2.23}{ GeV}$ was measured to experimentally validate and constrain theoretical predictions of the $\gamma Z$ box correction to the proton's weak charge and to probe the $F_{1,3}^{\gamma Z}$ structure functions. Final results will be presented and compared with theoretical predictions.   

NOPTREX, An Experiment to Search for T Violation in Polarized Neutron Optics : Resonance Measurements in $^{131}$Xe

Sensitive experimental searches for new sources of time reversal violation can uncover new phenomena beyond the Standard Model of particle physics and may be important for our understanding of the baryon asymmetry of the universe. We describe the concept behind an experimental search for a P-odd and T-odd term in the polarized neutron-polarized nucleus forward scattering amplitude \footnote{V.P. Gudkov, Physics Reports {\bf 212 }, 77-105 (1992).} planned by the NOPTREX collaboration which takes advantage of the approximate $10^{5}$\textemdash $10^{6}$ amplification of P-odd amplitudes in certain epithermal p-wave n-A resonances in nuclei such as $^{139}$La \footnote{V.P. Alfimenkov, L. Lason, Yu.D. Mareev, V.V. Novitsky, L.B. Pikelner, V.R. Skoy, M.I. Tsulaya, A.N. Chernikov, Phys. Atomic Nuclei {\bf 59}, 1861 (1996).}. This talk will discuss the preparation and design of an experiment to measure the spin-coupling ratio $\kappa$ in $^{131}$Xe at the ANNRI beamline at the Japan Proton Accelerator Complex (J-PARC) and the design of a low-noise current-mode neutron detector with near-unit efficiency and fast time response to resolve the resonance shapes.   

MuSun: a high precision measurement of the muon-deuteron capture rate

Muon capture on the deuteron is a weak two-nucleon process which can be related in a model-independent way to other processes of fundamental interest such as solar p-p fusion and neutrino-deuteron scattering. The MuSun experiment determines the disappearance rate of $\mu^{-}$ stopped in an ultra-pure deuterium target by measuring the time between the observed incident muons and their decay electrons. This rate is compared to the known $\mu^{+}$ decay rate to determine the capture rate with a goal of 1.5\% precision. The target operates as a time projection chamber, providing strict event selection by identifying muons within the target. The capture rate is 1000 times slower than free muon decay, demanding a ten ppm measurement of the decay rate, and thus $\sim10^{10}$ events with careful control of systematic effects. We have acquired $1.2\times10^{10}$ candidate events at the $\pi$E1 muon beam line at the Paul Scherrer Institute over two experimental campaigns. The experiment is a unique probe of the two-body weak current, and presents a number of challenges such as high target purity and identifying muon-catalyzed fusion events. Here we present the current status and recent analysis progress which will lead to a measurement of the capture rate to unprecedented precision.