Session L14: Minisymposium: Fundamental Symmetries III - Neutron

Probing Physics Beyond the Standard Model with Electric Dipole Moments

Electric dipole moments are sensitive to physics beyond the standard model. The CP phases in models beyond the standard model contribute to the electric dipole moments in various ways. We discuss the importance to measure different electric dipole moments since they are complemental to each other.   

Current Status of the TUCAN UCN Source and neutron EDM Experiment

The TRIUMF Ultra Cold Advanced Neutron (TUCAN) collaboration is developing a world-leading ultracold neutron (UCN) source for installation at TRIUMF.  The source combines proton spallation of a tungsten target with a liquid deuterium moderator and superfluid helium converter, and will produce UCN densities up to an order of magnitude higher than comparable sources.  The flagship experiment for this UCN source is a measurement of the neutron electric dipole moment (nEDM), while a second port will be open to proposals from users worldwide.  In the nEDM experiment, trapped UCN will be used to measure the neutron EDM using Ramsey's technique of separated oscillatory fields. With our newly upgraded source and carefully controlled magnetics, a statistical nEDM sensitivity of 10^−27 e⋅cm could be achieved in 400 days of data taking.  This presentation will present an overview of the UCN source and nEDM experiment installation at TRIUMF.   

Neutron EDM measurements using the Intense Ultracold Neutron Source at Los Alamos

The Electric Dipole Moment (EDM) of the neutron is a probe for the violations in the combined Charge-conjugate and Parity-reversal symmetry (CP). Many new theories beyond the Standard Model, which aim to unify the fundamental forces and solve the problem of Baryon Asymmetry of the Universe, also predict sizable EDM just lurking around the corner for discovery. However, the low density of UCN limits the worldwide progress of current nEDM experiments; existing facilities have not been able to deliver significantly higher UCN flux to experiments. We have been constructing an apparatus to take advantage of the opportunity of the unique LANL UCN facility: With the intense output from the LANL UCN source, the apparatus aims to perform a nEDM search at the 10$^{-27}$e-cm level of sensitivity. The upgraded LANL UCN source has enabled the most precise neutron lifetime measurement in the UCNtau experiment. A separate beamline has been added to allow the simultaneous operation of multiple experiments. The LANL nEDM experiment will apply Ramsey's separated oscillatory field method to measure the precession frequency of the neutron under a small, precisely controlled, static magnetic field. I will report the status of this effort.   

The Magnetically Shielded Room for the Neutron Electric Dipole Moment Experiment at LANL

The Standard Model predicts a neutron electric dipole moment (nEDM) on the order of 10^-32 e-cm. An nEDM greater than this predicted value would provide evidence for CP violating physics beyond the Standard Model. The experiment in development at the Los Alamos National Laboratory (LANL) ultracold neutron (UCN) source aims to measure the neutron EDM (with an uncertainty/to a sensitivity of) ~ 3x10^-27 e-cm using Ramsey's separated oscillatory field method. A measurement of this precision requires magnetic fields that are uniform to 0.1 nT/m and stable for hundreds of seconds. Therefore, a state-of-the-art magnetically shielded room (MSR) is needed. We have constructed an MSR with 4 mu-metal layers and one copper layer with an internal dimension of 2.3 m x 2.3 m x 2.3 m at the LANL UCN experimental area. This talk will discuss the design of the MSR, its current performance, and the results and stability of the degaussing procedure.   

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

The neutron electric dipole moment experiment at the Spallation Neutron Source (nEDM@SNS) will implement a novel method, which utilizes polarized ultra-cold neutrons (UCN) and polarized ³He in a bath of superfluid ⁴He, to place a new limit on the nEDM down to 2-3×10⁻²⁸ e·cm. The experiment will employ a cryogenic magnet and magnetic shielding package to provide the required magnetic field environment to achieve the proposed sensitivity. This talk will describe the design and implementation of a ³He polarimetry setup at the SNS to measure the monochromatic neutron polarization and transmission losses resulting from passage through the magnetic shielding and cryogenic windows. Preliminary results from monochromatic neutron polarization measurements as well as polarimetry component optimization will be presented.   

Development of a spin tracking simulation for the neutron electric dipole moment at the Spallation Neutron Source

The neutron Electric Dipole Moment experiment at the Spallation Neutron Source (nEDM@SNS) is being commissioned to reach sensitivities of ~ 3 x 10^-28 e•cm. Achieving this sensitivity requires tight control over systematic effects, which will be measured using the Systematics and Observational Studies at the PULSTAR reactor (SOS@PULSTAR) apparatus. SOS@PULSTAR shares many of the same features as nEDM@SNS but is much smaller, cooling and warming on the scale of weeks instead of months. For quickly gaining insights into how operating parameters affect our systematics it is then preferable to turn to simulation. We developed a package for simulating ultracold neutron and ³He spin dynamics using Graphics Processing Units (GPUs). Being able to leverage GPU resources significantly increases compute power, but makes development much more challenging. The entire code was written from scratch to make it as flexible as possible, requiring no external libraries. Shown here are results verifying the physics output of the code versus theory.   

Development of GPU-Accelerated Spin Tracking Algorithms for the nEDM@SNS Experiment

The neutron electric dipole moment (nEDM) experiment at the Spallation Neutron Source (nEDM@SNS) will utilize polarized ³He and ultra-cold neutrons (UCN) in a pair of measurement cells filled with superfluid ⁴He to place a new limit on the nEDM down to 2-3x10^-28 e·cm. Any deviations in the expected precession frequency of the ³He or UCN need to be understood from simulations to the nanohertz level to support the experimental precision goals. This necessitates large-scale datasets, ∼50 million particles each, to validate theory models within the experimental apparatus. Generating these datasets motivated the development of high-performance parallel integration routines that leverage graphics processing units (GPUs). A discussion of the adaptation of spin-integration to GPUs will be presented along with a quaternion-based spin integration algorithm optimized for the free-precession mode of the nEDM@SNS experiment.   

Speeding up spin dressing calculations for the nEDM@SNS experiment with Magnus integrators

The neutron electric dipole moment experiment at the Spallation Neutron Source (nEDM@SNS) will measure the nEDM by observing the spin-dependent rate of scintillation light produced by neutrons captured on helium-3 within a superfluid helium bath. In the critical dressing mode of this experiment, an oscillating magnetic field will dress the gyromagnetic ratios of neutrons and helium-3 to the same effective value, which will increase sensitivity to the nEDM by improving the signal-to-noise ratio. However, simulating the critical dressing mode is challenging with conventional numerical integration techniques, as the dressing field is both large in magnitude and rapidly varying. This, combined with the requirement that nEDM@SNS simulations be precise at the nanohertz level, leads to prohibitively expensive Monte Carlo simulations using traditional techniques. These simulations can be accelerated using the Magnus expansion to express the propagator over each time step as a product of one or more matrix exponentials. We compare this approach to conventional numerical integration schemes such as DOP853 and find that for certain problems Magnus integration can achieve similar accuracy results with longer step sizes and fewer function evaluations. We additionally investigate techniques to numerically integrate the pseudomagnetic field effect, which couples the helium-3 and neutron spins.   

neutron lifetime anomaly and test of mirror neutron oscillations

The neutron lifetime puzzle has been an evolving and complicated issue since high precision lifetime measurements began in the late 1980s. Probably the most notably anomaly is the 1% discrepancy between two sets of high precision measurements via the so-called "beam" and "bottle" approaches. Also related is the puzzle about the CKM unitarity. One of the possible explanations for such puzzles is from the newly proposed mirror matter model via ordinary-mirror neutron oscillations. Here we propose a set of feasible laboratory experiments to test concrete unique predictions of the mirror oscillation model, including measurement of neutron lifetime anomalies in narrow magnetic traps or under super-strong magnetic fields, and detection of unexpectedly large branching fractions of invisible decays of long-lived neutral hadrons. Their impact and implication on new physics beyond the Standard Model will be discussed.