Session L14: Fundamental Neutron Physics

The Nab Experiment: A Precision Measurement of Unpolarized Neutron Beta Decay

Neutron beta decay is one of the most fundamental processes in nuclear physics and provides means to uncover the details of the weak interaction. Neutron beta decay can evaluate the ratio of axial-vector to vector coupling constants in the standard model, $\lambda$ = $G_A$/$G_V$ , through multiple decay correlations. The Nab experiment will carry out measurements of the electron-neutrino correlation parameter $a$ with a precision of $\delta a/a$ = 10$\times 10^{-3}$ and the Fierz interference term $b$ to $\delta b$ = 3$\times 10^{-3}$ in unpolarized free neutron beta decay. These results aim to deliver an independent and competitive determination of $\lambda$ that will allow an evaluation of $V_{ud}$ and sensitively test CKM unitarity, independent of nuclear models. Nab utilizes a novel, long asymmetric spectrometer that guides the decay electron and proton to two large area silicon detectors in order to precisely determine the electron energy and proton momentum. The Nab apparatus is under installation on the Fundamental Neutron Physics Beamline at the Spallation Neutron Source at Oak Ridge National Lab. We present the motivation, goals, and principles of the Nab experiment.   

Progress on the Design, Construction and Test of the Full Size Magnet for the SNS Neutron Electric Dipole Moment Search

The SNS nEDM project aims to search for a neutron electric dipole moment (nEDM) at a goal sensitivity of 3x10^-28 e-cm at the Spallation Neutron Source (SNS) of Oak Ridge National Laboratory (ORNL). The nEDM signal will appear as a change of the procession frequency of polarized ultracold neutrons (UCN) induced by the application of a high electric field. A uniform magnetic field over the UCN storage volume of 0.4 m long with a gradient less than 3 ppm/cm is crucial to suppress the false nEDM signal due to the geometric phase effect, and the field strength will be monitored by comagnetometer He-3. The magnetic fields will be generated by superconducting cosine-theta coils of 1.5—2 m in diameter, enclosed in a nearly-hermetic superconducting lead shield.  In this talk, we will present the progress of design, construction and tests of the full size magnets and magnetic shielding as well as the cryogenic systems. The challenges and solutions will be discussed.    

3He Spin Transport Magnet Development for the SNS nEDM Experiment

Measurements of the neutron electric dipole moment (nEDM) are one of the most important tests of the parity and time reversal symmetries with implications both to baryogenesis in the early universe and fundamental physics. The SNS nEDM experiment aims to make a measurement to an accuracy of $3\times10^{-28}$ e-cm, 100 times better than previously existing measurements that have set an upper limit on the value of the nEDM. One of the methods to be used to meet the goal accuracy will be to use polarized helium-3 both as a co-magnetometer and as part of the neutron polarization measurement process.


A novel series of magnets are planned to be used to transfer the polarized helium-3 from the atomic beam source into the nEDM measurement cell. As part of the magnet development process a re-configurable mu-metal magnetic shielding enclosure is planned. This enclosure will allow different magnet designs and configurations to be tested in a similar magnetic environment to the magnetic shielding enclosure that will be used in the experiment. I will discuss the design and function of the magnetic test enclosure.
  

Progress toward a new room temperature neutron electric dipole moment search at the Los Alamos Neutron Science Center

A discovery of the neutron's permanent electric dipole moment larger than the standard model prediction of d_n ≈ 10^-31 e·cm would signal a new source of CP-violation and help explain the matter-antimatter asymmetry in the universe. Tightening the limits on d_n constrain extensions to the standard model in a complementary fashion to the atomic and electron EDM searches.  The recent upgrade of the Los Alamos ultracold neutron source makes it possible for a new room temperature search with the statistical reach to improve upon current limits by a factor of 10 or more. During the 2018 LANSCE cycle a prototype apparatus was used to demonstrate the capability to transport and manipulate polarized neutrons and perform Ramsey and Rabi sequence measurements. I will report on the measurements made over the last year, efforts underway to upgrade the prototype chamber, and possible future upgrades of the ultracold neutron source.