Search for the neutron electric dipole moment at PSI: The n2EDM Project of the nEDM collaboration

In order to address this fundamental question, which has perplexed cosmologists for half a century, we need to look beyond our Standard Model (SM) of particle physics. Some of the most sensitive probes we have of such “new” physics arise from non-accelerator experiments. One such is the search for an electric dipole moment (EDM) of the neutron. EDMs violate both parity and time-reversal symmetries, and they therefore provide some of the tightest constraints upon models beyond the SM that attempt to reconcile the low level of CP violation observed in K and B systems with the large baryon asymmetry of the Universe. The current world limit of 1.8 x 10-26 e.cm, set in 2020 by the nEDM collaboration, already requires considerable fine-tuning of MSSM parameters - the so-called "SUSY CP problem". The measurement is carried out by using NMR to determine the Larmor precession frequency, in parallel and antiparallel magnetic and electric fields, of ultracold neutrons trapped in a storage cell - a sort of atomic clock, but using neutrons instead of atoms. A change in frequency with applied electric field is the signature of an EDM. The level of precision is astonishing: the system can detect an energy-level splitting of 10-21 eV, and yet it is sensitive to aspects of physics at energy scales well beyond that achievable at the LHC. Watch video of artists: Capturing nEDM in collaboration with the ETHZ Department of Architecture.

The neutron electric dipole moment (nEDM) is a measure for the distribution of positive and negative charge inside the neutron. A finite electric dipole moment can only exist if the centers of the negative and positive charge distribution inside the particle do not coincide. So far, no neutron EDM has been found. Read more in the Wikipedia entry...

After running nEDM with the world’s best sensitivity in 2015/16, see PRL, we are constructing n2EDM for at least an order of magnitude improvement. The technical design of our new n2EDM apparatus has been published in Physical Review A. 2021; 103(6): 062801

CAD image of the n2EDM apparatus.
CAD image of the n2EDM apparatus without the surrounding active magnetic shield (AMS) and the so-called "thermohouse", the outside layer for thermal insulation.


MSR n2EDM
The magnetically shielded room (MSR) of the n2EDM apparatus, commissioned in 2020.



n2EDM opened
The core of the n2EDM apparatus with all doors (MSR, B0, vacuum tank) open, status October 2024.



n2EDM opened
The n2EDM experiment in an opened state in 2024, and the central stack rolled out.
Segarra figures

Achieving ultra-low and -uniform residual magnetic fields in a very large magnetically shielded room for fundamental physics experiments

n2EDM is the current state of the art experiment carrying out a high-precision search for an electric dipole moment of the neutron at the ultra-cold neutron source of PSI. In order to reach it’s incredible precision of 10-27 e cm, a stable and uniform magnetic environment is critical. Thus, shielding the experiment from external magnetic flux and preparing a pristine magnetic environment is crucial. To achieve this, n2EDM uses both passive and active magnetic shielding components. External, or residual, magnetic field contributions must be near-zero, and can be achieved via “degaussing” the experiment’s passive magnetic shielding. Degaussing reduces, ideally “erases”, the residual magnetization of a material. In this work, we greatly improved the degaussing procedure of n2EDM, reducing the residual magnetic field by a factor of two, improving its uniformity, and all while taking less time and dissipating less heat.

hero_psi_grafiken_artikel_01_221219-1-1.jpg
•Grossforschungsanlagen
•17 min

Mehr Licht ins Dunkel

Teilchenphysik
IMPACT
CHRISP

Am PSI wollen Forschende mithilfe der Grossforschungsanlagen die letzten Lücken im Standardmodell der Physik schliessen.

aux-origines-de-la-matiere

CNRS movie on n2EDM

Our French collaborators and CNRS produced an excellent short movie about our common n2EDM experiment. The apparatus is currently being set up in

UCN Area South. The collaboration is on track for commissioning of the apparatus with neutrons towards the end of 2022.

Georg Bison (links), Bernhard Lauss (Mitte) und Klaus Kirch
•Grundlagen der Natur
•10 min

Magnetisch abgeschirmt vom Rest der Welt

Teilchenphysik
CHRISP

Am Paul Scherrer Institut PSI haben Forschende gemeinsam mit einer Firma einen Raum konstruiert, der einer der am besten magnetisch abgeschirmten Orte auf der Erde ist. Mit seiner Hilfe wollen sie einige der letzten Rätsel der Materie lösen und der Antwort auf eine fundamentale Frage näher kommen: Warum gibt es überhaupt Materie und damit auch uns selbst?

Magnetically shielded room with ultralow magnetic field in the central 25 m3 volume finished commissioning, see final report in Rev. Scient. Instr. 93, 095105 (2022). Read also PSI highlight… 
New limit on the nEDM: The nEDM collaboration at PSI has published in Physical Review Letters the most sensitive measurement of the neutron EDM to date: dn= (0.0 ± 1.1stat ± 0.2sys) × 10-26 e·cm. Read on… 

Constructing n2EDM After running nEDM with the world’s best sensitivity in 2015/16, we are constructing n2EDM for an order of magnitude improvement.
ERC Starting Grants Awards Two members of our collaboration, Guillaume Pignol and Florian Piegsa, have been awarded ERC Starting Grants for research projects connected to the neutron EDM search.