Session PJ: Electroweak III

Overview of the Science Program at the LANL Ultracold Neutron Source

Los Alamos National Laboratory (LANL) currently operates a proton-beam-driven solid-deuterium-based ultracold neutron (UCN) source. It was originally built to provide UCN to the UCNA experiment, an experiment to measure the angular correlation between the neutron spin and electron emission in polarized beta decay of free neutrons. The science program at the LANL UCN source has since significantly grown. It now provides UCN to various activities including a suite of neutron beta decay experiments, R{\&}D for the SNS nEDM experiment, detector development for the Nab experiment at the SNS, development of a new nEDM experiment at LANL, and a study of material damage due to neutron induced fission fragments. In addition, the UCN source is in the process of being upgraded and an active research program on UCN source and guide technology is taking place. In this talk, an overview of the science program at the LANL UCN source will be given.   

Upgrades to the ultracold neutron source at the Los Alamos Neutron Science Center

The spallation-driven solid deutrium-based ultracold neutron (UCN) source at the Los Alamos Neutron Science Center (LANSCE) has provided a facility for precision measurements of fundamental symmetries via the decay observables from neutron beta decay for nearly a decade. In preparation for a new room temperature neutron electric dipole moment (nEDM) experiment and to increase the statistical sensitivity of all experiments using the source an effort to increase the UCN output is underway. The ultimate goal is to provide a density of 100 UCN/cc or greater in the nEDM storage cell. This upgrade includes redesign of the cold neutron moderator and UCN converter geometries, improved coupling and coating of the UCN transport system through the biological shielding, optimization of beam timing structure, and increase of the proton beam current. We will present the results of the MCNP and UCN transport simulations that led to the new design, which will be installed spring 2016, and UCN guide tests performed at LANSCE and the Institut Laue-Langevin to study the UCN transport properties of a new nickel-based guide coating.   

Measurement of spin flip probabilities for ultracold neutrons on guide materials

Ultracold neutrons (UCNs) are defined as neutrons with kinetic energy sufficiently low so that they can be confined in a material bottle. UCN sources are used in many facilities worldwide to pursue some of the most profound questions in fundamental physics. UCN guides, which transport UCNs from the source to experiments, play a crucial role in achieving high UCN density in an experimental apparatus. In some cases, UCN guides are also required to transport spin polarized UCNs, and therefore the probability of spin flip upon UCN interaction is an important property characterizng UCN guide materials. We have studied the depolarization property of a new nickel based UCN guide coating material. In this talk, the purpose, method, and results of the experiment will be presented and the implication of the results on the depolarization mechanism will be discussed.   

Feasibility of Parity-Violating Electron Scattering Experiments Below 1 GeV Beam Energy with a Toroidal Spectrometer

The next generation of high precision parity-violating electron scattering experiments could potentially make use of a toroidal spectrometer to perform additional measurements of the proton's weak charge ($Q^{p}_{w}$) using a hydrogen target, a test of the Standard Model using a carbon target as well as possibly studying the neutron skin of heavier nuclei. I will present the results of recent Geant4 Monte-Carlo studies performed to test the feasibility of such a toroidal spectrometer at beam energies below 1 GeV employing a concept similar to that used by the recent JLab $Q_{weak}$ measurement [Nucl. Instrum. Meth. A781 (2015) 105-133]. It appears that given sufficient beam time such a measurement could be complementary to the JLab measurement, but at a significantly lower $Q^{2}$. The feasibility of measuring the neutron skin using such a spectrometer will also be discussed. The key issue for this latter type of measurement is the ability to achieve the necessary resolution to separate the elastic and first excited state.   

New Measurements of the EMC Effect and Short Range Correlations at JLab Hall C at 11 GeV

The nuclear dependence of the Deep Inelastic Scattering (DIS) cross section (known as the EMC effect) has shown conclusively that the distribution of quarks in a nucleus is modified when compared to the deuteron. Short Range Correlations arise from hard interactions between nucleons inside the nucleus, which create high momentum tails on nucleon momentum distributions. The observation that the size of the EMC effect is correlated with the number of SRC NN pairs in a nucleus suggested a possible origin of the EMC effect. While the observed correlation is compelling, the correlation merits more investigation by adding additional nuclei and improving the precision on existing measurements. Jefferson Lab experiments E12-06-105 and E12-10-008 aim to do just that, making measurements of electron scattering cross section ratios in the DIS regime and at $x>1$ for a large body of nuclei. As part of the initial commissioning of the new SHMS spectrometer in Hall C, a subset of the proposed measurements will add $^{10}$B and $^{11}$B, as well as improving existing measurements of $^{27}$Al. Commissioning plans and details of early measurements will be discussed.   

Measurement of Neutron Emissions from Nuclear Muon Capture

The AlCap collaboration is studying particle emission after muon capture on Al and Ti nuclei. Proton and neutron emission are an important source of accidental activity in the Mu2e and COMET experiments, which will search for charged lepton flavor violation (CLFV) in neutrino-less muon to electron conversion in the field of an atomic nucleus. A recent experiment was completed at the high intensity piE5 beamline at the Paul Scherrer Institute (PSI) focusing on neutron and gamma emissions from Al. AlCap expects to obtain the bound muon lifetime, the low-energy neutron spectrum, and the neutron emission rates per muon capture. The current state of the analysis will be presented.   

Search for exotic short-range interactions using paramagnetic insulators

We describe a proposed experimental search for exotic spin-coupled interactions using a solid-state paramagnetic insulator. The experiment is sensitive to the net magnetization induced by the exotic interaction between the unpaired insulator electrons with a dense, nonmagnetic mass in close proximity. An existing experiment has been used to set limits on the electric dipole moment of the electron by probing the magnetization induced in a cryogenic gadolinium gallium garnet sample on application of a strong electric field. With suitable additions, including a movable source mass, this experiment can be used to explore ``monopole-dipole'' forces on polarized electrons with unique or unprecedented sensitivity. The solid-state, nonmagnetic construction, combined with the low-noise conditions and extremely sensitive magnetometry available at cryogenic temperatures could lead to a sensitivity over 10 orders of magnitude greater than exiting limits in the range below 1 mm.   

Search for tensor-like couplings in the $\beta $-decay of laser trapped $^{6}$He

The beta-neutrino angular correlation in nuclear beta decay can reveal the nature of the weak interaction. The case of $^{6}$He is particularly sensitive to test for tensor contributions by measuring the corresponding angular correlation parameter $a_{\beta \nu }$. Trapping techniques such as magneto-optical traps (MOT) combined with recoil ion momentum spectroscopy are powerful tools which allow to measure $a_{\beta \nu}$ with high precision. The experiment, located at the University of Washington, takes advantage of the tandem Van de Graaff accelerator to produce up to 2x10$^{10} \quad^{6}$He/s. A double-MOT setup has been optimized to trap and detect beta-recoil-ion coincidences at a rate of a few Hertz. Systematic effects have been investigated in details and major effort has been put to limit their contribution to less than 1{\%} of $a_{\beta \nu }$. The first goal of this experiment is to measure $a_{\beta \nu }$ with this 1{\%} uncertainty and use this set of data to guide further improvements with the goal to bring the uncertainty to the 0.1{\%} level. The performances of the trap setup, preliminary coincidence data, and studies of systematic uncertainties will be presented. This work is supported by DOE, Office of Nuclear Physics, under contract nos. DE-AC02-06CH11357 and DE-FG02-97ER41020.   

Data analysis and systematic studies for the He-6 experiment

The He-6 experiment at the University of Washington aims to precisely measure the beta-neutrino angular correlation ($a_{\beta\nu}$) in the beta decay of He-6, a parameter that is particularly sensitive to tensor-like currents in the electroweak interaction. The experiment is based on a coincidence detection of the beta and recoil ion emitted from laser trapped He-6 and seeks to ultimately measure $a_{\beta\nu}$ to the $0.1\%$ level. Monte-carlo simulations of the decay and detection scheme are essential to analyze the data and have been extensively used to quantify the effects of systematic uncertainties. Major efforts have been put in to limit their contributions to less than $1\%$ of $a_{\beta\nu}$, the first goal of the experiment. This set of data will guide further improvements of the experiment towards the $0.1\%$ level measurement of $a_{\beta\nu}$. The data analysis procedures and the current status of the experiment, including the achieved and projected systematic and statistical uncertainties, will be presented.