The search for the neutron electric dipole moment at the Paul Scherrer Institute

Abstract

International audienceThe measurement of the neutron electric dipole moment (nEDM) constrains the contribution of CP-violating terms within both the Standard Model and its extensions. The experiment uses ultracold neutrons (UCN) stored in vacuum at room temperature. This technique provided the last (and best) limit by the RAL/Sussex/ILL collaboration in 2006: dn < 2:9 × 10-26 e cm (90% C.L.). We aim to improve the experimental sensitivity by a factor of 5 within 2-3 years, using an upgrade of the same apparatus. We will take advantage of the increased ultracold neutron density at the Paul Scherrer Institute (PSI) and of a new concept including both, external magnetometers and a cohabiting magnetometer. In parallel, a next generation apparatus with two UCN storage chambers and an elaborate magnetic field control is being designed aiming to achieve another order of magnitude increase in sensitivity, allowing us to put a limit as tight as dn < 5 × 10-28 e cm (95% C.L.), if not establishing a finite value

Comparison of ultracold neutron sources for fundamental physics measurements

“…The running nEDM experiment at PSI [11] employs a deuterated polystyrene (dPS) coating in the precession chamber with a Fermi potential of 161 neV [75] and a volume of 21 000 cm 3 . The mobility of the setup and hence a necessary certain robustness of construction was an additional requirement.…”

“…This was for a long time the only source with sufficient UCNs available. At the beginning experiments concentrated on measurements of the free neutron lifetime (τ n ) [5][6][7][8] and on the search for a permanent electric dipole moment of the neutron (nEDM) [9][10][11][12][13]. In addition to the fundamental particle aspect, τ n is also an important quantity in understanding the primordial nucleo-synthesis and contributes presently one of the larger uncertainties to its description [14].…”

Measures to reduce the residual field and field gradient inside a magnetically shielded room by a factor of more than 10

“…Here, a new degaussing system and equipment selection led to a significant improvement of the experiment [2].…”

“…Examples of such high resolution experiments are spin experiments with 3 He or 129 Xe [1], neutrons [2,3], ultra-low field NMR [4], or MRI [5] or biomagnetic recordings [6]. Each of these experiments has special magnetic requirements, like low residual magnetic fields, low magnetic field gradients and/or low residual field drifts.…”

Large magnetic shielding factor measured by nonlinear magneto-optical rotation

“…−27 e-cm [1,2,3,4,5,6,7,8,9]. In the previous best experiment [10], which discovered d n < 2.9× 10 −26 e-cm, effects related to magnetic field homogeneity and instability were found to dominate the systematic error.…”