Session L11: Tests of Time Reversal Violation II

Live

The neutron's permanent electric dipole moment (nEDM) measures the separation of charges inside a neutron and violates both parity (P) and time-reversal (T) invariance. T-violation is very small in the standard model, leading to an nEDM of about 10$^{\mathrm{-31}}$ e-cm. Yet, T-violation is a key ingredient for the dynamical generation of the cosmic matter-antimatter asymmetry, a major unanswered puzzle in the SM. If physics beyond the standard model (BSM) appears at an energy scale of 1--100 TeV, as widely expected from theoretical considerations, then one quite naturally expects nEDMs within a few orders of magnitude of the current limit of 3\texttimes 10$^{\mathrm{-26}}$ e-cm (90 C.L.). The recent successful upgrade of the LANL Ultra-Cold Neutron (UCN) source, which more than quadrupled its performance, has created an exciting opportunity to advance the US search for the nEDM. The LANL nEDM experiment, based on the proven Ramsey's separated oscillatory field method at room temperature, aims to perform an nEDM search at a sensitivity of a few times 10$^{\mathrm{-27}}$ e-cm. In this talk, the current status of the LANL nEDM experiment, including the details of the engineering design, status of the apparatus fabrication, and commissioning schedule, will be presented.   

Live

Permanent electric dipole moments represent a prospective avenue for the discovery of beyond standard model physics. The advent of experimental techniques using stored ultracold neutrons (UCNs) has placed the neutron electric dipole moment (nEDM) at the forefront of EDM searches. The current experimental upper limit for the nEDM is $d_n < 3\times10^{-26}$ e-cm (90\% CL). The neutron EDM search to be conducted at Los Alamos National Laboratory (LANL) aims to advance the experimental measurement of the nEDM by an order of magnitude. Achieving the goal sensitivity of $3\times10^{-27}$ e-cm requires a highly uniform $B_0$ holding field as well as efficient transport of UCN polarization from the neutron source into the storage volume. This talk will discuss the design of the $B_0$ coil and the spin transport coil system for the LANL-nEDM experiment.   

Live

A non-zero electric dipole moment (EDM) of a fundamental particle violates the time-reversal symmetry, implying a violation of the combined charge conjugation and parity (CP) symmetry. The current experimental limit of nEDM provides stringent constraints on new sources of CP violation beyond the Standard Model of particle physics. The nEDM experiment at Los Alamos National Laboratory (LANL), with its upgraded ultracold neutron source, aims to improve this measurement to $3.0\times 10^{-27} e \cdot cm$, which is an order of magnitude smaller than the current best limit. In controlling the magnetic field background to achieve this sensitivity, we plan to implement mercury (Hg)---both as the co-magnetometer and as an external magnetometer. The latter would be the first implementation of its kind for nEDM applications. I will describe the details about the magnetometers, and present methods utilizing the magnetometers to improve the uniformity of the magnetic field and to quantify the geometric phase systematic error.   

On Demand

The charge of the neutron is an extremely well measured number, and is consistent with zero. Nonetheless, motivations for the existence of minicharges questions charge conservation and thereby the neutrality of neutrons. Room temperature neutron electric dipole moment (nEDM) experiments employ the Ramsey technique of separated oscillating fields to search for a permanent EDM of the neutron. Due to a large geometric phase effect coming from the magnetic field gradients that the UCNs experience, usually the EDM is extracted using a crossing point analysis which involves measuring the EDM at well known non-zero gradients, and then extracting the EDM at zero gradient. One of the free parameters in the crossing point analysis is the center of mass offset between the ensemble of UCNs and the cohabiting $^{199}$Hg atoms, which is used as a co-magnetometer. Given that the UCNs are stored under the influence of a strong electric field, even though the precession frequency of the neutrons extracted using the Ramsey technique has no dependence on the charge of the neutron, the center of mass offset in the crossing point analysis may be dependent on the charge of the neutron. The sensitivity reach of this technique at the leading nEDM experiments will be presented.   

Measurement of Neutron Polarization and Transmission for the SNS nEDM Experiment.

The existence and size of a neutron electric dipole moment (nEDM) remains an important question in particle and cosmological physics. The SNS nEDM experiment proposes a new limit for nEDM search by using ultra-cold neutrons (UCN) in a bath of superfluid helium. The experiment uses polarized 8.9{\AA} neutrons to create polarized UCN in situ in superfluid helium via superthermal downscattering. This process requires the 8.9{\AA} neutrons to retain their polarization as they pass through the magnetic shielding and nEDM cryostat windows. This talk will describe a setup to measure the neutron polarization loss from the magnetic shielding and cryostat windows.