Particles and fundamental interactions


Measurement of the electric dipole moment of the neutron

In 1967, A. Sakharov formulated three conditions necessary for the appearance of matter in our Universe: an out-of-equilibrium phase in the Universe evolution, a breaking of C and CP symmetries and the non-conservation of the baryon number B. Two of these conditions are not present in the Standard Model of Particle Physics. On the other hand, some alternative models, describing a physics known as beyond the standard model (BSM), offer viable scenarios while satisfying the Sakharov’s conditions. Measurements of electric dipole moments allow to constrain the BSM models currently in competition.

More precisely, measurements of electric dipole moments (EDM) of elementary particles or composite systems (electron, neutron, Hg, etc.) probe any possible mechanisms of CP symmetry breaking. A non-zero measurement indicates a CP violation. This type of measurements began in 1957 with a first estimate of the neutron EDM. They have been pursued until today without any team succeeding in demonstrating an EDM other than zero (out of about twenty systems studied).

In this context, the international nEDM collaboration (fifteen laboratories), seeks to measure the electric dipole moment of the neutron at the Paul Scherrer Institute (PSI) in Switzerland. The gain in sensitivity, an order of magnitude, will be achieved thanks to the high intensity ultra-cold neutron source at PSI and the construction of a new spectrometer. 

The project consists of two phases, nEDM and n2EDM: the first was completed. It led to the publication of the most accurate measurement of neutron EDM in 2020 (Phys. Rev. Lett. 124, 081803 (2020)). The second phase began (n2EDM). A new spectrometer is under construction. The success of the experiment relies on different highly performant components of the apparatus: a magnetically shielded room whose performances are unique in the world with such dimensions, the production of an extremely uniform magnetic field, the control of the magnetic field (online with a complex system of magnetometers and offline with a sophisticated device for the mapping of the field), the installation of a large non-magnetic vacuum chamber etc… Data taking will begin in 2023 and last 4 years in order to achieve a sufficient sensitivity.

Within this project, the LPC is in charge of the neutrons detection, the analysis of their polarization, the design of the coils generating the magnetic fields and the construction of the non-magnetic vacuum chamber.

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Aux origines de la matière, de Pierre De Parscau © CNRS – 2022

For more details on the n2EDM project look at:

  • Tracking down the mystery of matter
  • The design of the n2EDM experiment - Eur. Phys. J. C (2021) 81:512
  • Measurement of the permanent electric dipole moment of the neutron C. Abel, S. Afach, N.J. Ayres et al. Phys. Rev. Lett. 124 (2020) 081803 10.1103/PhysRevLett.124.081803
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