page_title: Neutron electric dipole moment
section_title: History
hierarchical_section_title: Neutron electric dipole moment / History
caption_attribution_description: History of the upper limit on the electric dipole moment of the neutron. In addition, the predictions coming from Supersymmetry and the Standard Model are given.
context_page_description: The neutron electric dipole moment 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. The current best measured limit for dₙ is ×10⁻²⁶ e⋅cm.
context_section_description: The first experiments searching for the electric dipole moment of the neutron used beams of thermal (and later cold) neutrons to conduct the measurement. It started with the experiment by Smith, Purcell, and Ramsey in 1951 (and published in 1957) obtaining a limit of |dₙ| < 5×10⁻²⁰ e⋅cm . Beams of neutrons were used until 1977 for nEDM experiments. At this point, systematic effects related to the high velocities of the neutrons in the beam became insurmountable. The final limit obtained with a neutron beam amounts to |dₙ| < 3×10⁻²⁴ e⋅cm.
  After that, experiments with ultracold neutrons took over. It started in 1980 with an experiment at the Leningrad Nuclear Physics Institute (LNPI) obtaining a limit of |dₙ| < 1.6×10⁻²⁴ e⋅cm . This experiment and especially the experiment starting in 1984 at the Institut Laue-Langevin (ILL) pushed the limit down by another two orders of magnitude yielding the above quoted best upper limit in 2006, revised in 2015.
  During these 50 years of experiments, six orders of magnitude have been covered, thereby putting stringent constraints on theoretical models.
caption: Measured upper limits of the neutron EDM. Given are also the predictions stemming from Supersymmetry and the Standard Model
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