A new cryogenic apparatus to search for the neutron electric dipole moment

Abstract

A cryogenic apparatus is described that enables a new experiment, nEDM@SNS, with a major improvement in sensitivity compared to the existing limit in the search for a neutron Electric Dipole Moment (EDM). This apparatus uses superfluid 4 He to produce a high density of Ultra-Cold Neutrons (UCN) which are contained in a suitably coated pair of measurement cells. The experiment, to be operated at the Spallation Neutron Source at Oak Ridge National Laboratory, uses polarized 3 He from an Atomic Beam Source injected into the superfluid 4 He and transported to the measurement cells where it serves as a co-magnetometer. The superfluid 4 He is also used as an insulating medium allowing significantly higher electric fields, compared to previous experiments, to be maintained across the measurement cells. These features provide an ultimate statistical uncertainty for the EDM of 2 − 3 × 10 −28 e-cm, with anticipated systematic uncertainties below this level.

Precision experiments with muons and neutrons

“…In particular, the nEDM experiment has collected data [44] in 2015-2016 at the PSI UCN source, which will result in a marginally improved measurement of the neutron EDM (the analysis is still ongoing at the time of writing). Other ongoing nEDM projects [45][46][47][48][49][50][51] are listed in Table 1, they are all at a different stage of readiness and they aim at an improvement in sensitivity by a factor 10 to 100 compared to the previous measurement [38] in the next decade. These projects to measure the neutron EDM are an important part of a global search for fundamentals EDMs.…”

A new neutron lifetime experiment with cold neutron beam decay in superfluid helium-4

“…These technologies are under development and tests by the SNS nEDM collaboration. [37] With a loss probability per bounce of f (E U CN ) = 10 −5 , the same requirement as the SNS nEDM UCN storage cells [38], the time constant due to wall loss τ wall is about 1672.0 s . τ up is the loss rate due to upscattering of neutrons by quasi-particles, phonons and rotons, in superfluid helium.…”

Measurement of Neutron Polarization and Transmission for the Neutron EDM at SNS Experiment

“…The nEDM@SNS experiment will employ a multi-pronged strategy to shield against external fields: external field cancellation coils, room-temperature shielding and superconducting shielding [7]. As shown in figure 2, the nEDM magnet cryostat magnet, which surrounds the measurement cells, is made of several concentric layers.…”

“…Prior to the actual experiment, neutron polarization and transmission measurements will be performed at the SNS Beamline 13A [8]. The neutron beam will traverse the nEDM magnet system via beam windows made of thin metal on the room-temperature vacuum shell and cryogenic thermal shielding, then superconducting Pb, used for improving the magnetic uniformity and for shielding against external AC magnetic fields, then low-cobalt Metglas as a ferromagnetic flux return, then two Si windows serving as the entrance to the central detector volume and a deuterated poly-methyl methacrylate (dPMMA) window of the measurement cell [7]. The objective is to determine the neutron polarization loss and transmission of each of the windows, especially Metglas and Pb, in a similar magnetic field environment of the actual nEDM experiment.…”