Session MG: Mini-Symposium: Low Energy Probes of New Physics IV

Field Gradient Cancellation Technique for the LANL nEDM Experiment

The search for the permanent electric dipole moment of neutron (nEDM) has the goal of exploring sources of beyond standard model CP violating physics. The room temperature LANL nEDM experiment was proposed to push the upper limit of nEDM to $3 \times 10^{-27}$ e$\cdot$cm (68\% confidence level). Achieving this substantially high sensitivity requires a highly uniform magnetic field which lowers the systematic uncertainty associated with the field non-uniformity below the precision goal of the experiment. The geometric phase false EDM effect is the primary consideration for the field uniformity requirement and it demands that the magnetic field gradients to be controlled below $0.3$ nT$\cdot$m$^{-1}$ at the nominal $B_0$ field of 1 $\mu$T. I will describe the technique and the design of the proposed gradient coils to cancel first order and higher order magnetic field gradients to achieve the field uniformity requirements of the LANL nEDM experiment.   

Magnetic Coil System for the LANL nEDM Experiment

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 permanent electric dipole moment searches. The current experimental upper limit for the nEDM is $d_n < 1.8\times10^{-26}$ 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 to a sensitivity of $3\times10^{-27}$. Reaching the proposed sensitivity requires precise magnetic field control, specifically a highly uniform $B_0$ holding field, as well as efficient transport of UCN polarization from the polarized neutron source into the storage volume. A series of magnetic coils will be employed to meet the specifications for field uniformity and polarization transport. This talk will discuss the design of the magnetic coil system for the LANL-nEDM experiment.   

The magnetometer system planned for the neutron electric dipole moment (nEDM) experiment at LANL

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$^{199}$)---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 state-of-the-art techniques to instrument these magnetometers, including the laser system to polarize and detect the spin-dependent interactions and the making of these highly specialized optical cells to contain the mercury vapors.   

Magnetic Field Simulations for the n2EDM Experiment at Paul Scherrer Institute

Precision spin-precession experiments like searches for the neutron electric dipole moment (nEDM) place stringent requirements on the magnetic field. We present a global simulation of a magnetic field map in and around the six-layer magnetic shield room (MSR) of the n2EDM experiment, a large multinational endeavour to measure the nEDM with a statistical sensitivity of $1\times10^{-27}~e\cdot$cm. In addition to the precession field and spin transport coils, our simulation takes into account nearby superconducting coils and a high-performance active magnetic shielding system. We describe techniques used to characterize measurements of these coils, integrating them with backgrounds fields into the environment of the MSR. This simulation will be used to optimize spin transport of UCNs in and out of the Ramsey measurement cell, and to study the effect of external fields on the experimental sensitivity.   

Electromagnetic device for MEOP polarization of 3He at PULSTAR

We present the design of electromagnetic spin transport coils for the Syste-matic Operational Studies apparatus at the PULSTAR reactor at North Caro-lina State University, which will investigate techniques to improve NMR techni-ques needed for the nEDM experiment at the Spallation Neutron Source. These coils will guide the spin of the 3He co-magnetometer from the Metastability Ex-change Optical Pumping (MEOP) polarizer to inside the measurement cryostat. The device consists of an empty cylinder coil with circular covers on the top and bottom and which provides a uniform 5 G magnetic eld everywhere inside the cylinder. The nal design of this device is presented with high magnetic eld uniformity inside the ducial volume, satisfying two dierent constraints on eld gradients corresponding to separate relaxation of polarization requirements .   

3He Spin Transport in the SNS nEDM Experiment

Towards achieving a precision of $3 \times 10^{-28} e\cdot cm$, the SNS nEDM experiment will use a $^3$He comagnetometer, polarized by an external atomic beam source. The challenge of preserving the $^3$He polarization involves collecting spins from the ABS quadrupole field and tapering the field from 1 T down to 30 mG while passing through three layers of a magnetic shield enclosure. Using the magnetic scalar potential, we designed a system of coils to achieve an adiabatic magnetic field profile matching these requirements with less than $1\%$ loss in polarization. I will describe the design of our plans to characterize these coils.   

A magnetic shield for encompassing spin transport coils

High precision spin-precession experiments (EDMs for example) require weak (10-50 mG) spin transport fields that are easily sensitive to any external field, especially earth's magnetic field. These magnetic coils must be surrounded by a magnetic shielded environment, for which specialized equipment is not available, to avoid these external fields. I will describe a flexible mu-metal two-layer magnetic shield with a uniform interior field and a X-Y-Z mapping system within the shield, that provides a full 3d field mapping of the internal coils. The stages in the mapping system move by motors, spinning various pulleys connected to chains, outside the shield using a low-cost assembly of commodity nonmagnetic parts together with 3-d printed mechanisms and mounting hardware.   

A Magnetic Cloak for The SNS nEDM Experiment

High precision measurements such as the search for the neutron Electric Dipole Moment (nEDM) require an extremely uniform magnetic field in order to reduce the false nEDM signals. The SNS nEDM experiment will use a superconducting shield to reduce magnetic fluctuations in the measurement. The superconductor is surrounded by a uniform magnetic field for He-3 spin transport and to increase the uniformity of the measurement field. We will present the design for a magnetic cloak, which prevents distortion of the uniform external field by the superconductor. We also present the design of a modified cos-theta coil to test the cloak in a uniform magnetic field.   

Fitting strategies for the SNS nEDM experiment in the presence of time-varying magnetic fields

The Spallation Neutron Source (SNS) neutron electric dipole moment experiment (nEDM) performs a Larmor spin precession technique on ultracold neutrons and $^3$He. The spin-dependent interaction of the ultracold neutrons and polarized $^3$He co-magnetometer atoms will produce sinusoidal oscillations in the scintillation light that is sensitive to the difference in the neutrons' and 3He atoms' precession frequencies, $f_{3n}$. If the neutron EDM is non-zero, the value of $f_{3n}$ will shift upon reversal of the relative directions of the magnetic and electric fields. The precession frequency of the $^3$He will be measured via detection of the precessing 3He magnetization in SQUID pickup loops. We carried out simulations of the SQUID and scintillation rate signals in the presence of time-varying magnetic fields and then performed a global fit by minimizing the $\chi^2$ made up of the two signals. We present different fitting strategies for a global fit to the scintillation light and the 3He co-magnetometer signal in the presence of time-varying magnetic fields.