A measure of charge distribution asymmetry in particles like protons and deuterons, sensitive to new physics beyond the Standard Model.The electric dipole moment (EDM) of a particle is a measure of how its positive and negative charge distributions are separated, even when the particle is at rest. For a perfect sphere of charge, the EDM would be zero, but any asymmetry—like a slight 'pear shape'—produces a nonzero EDM. In the Standard Model of particle physics, EDMs are predicted to be extremely tiny, but many theories beyond the Standard Model (such as supersymmetry) predict much larger values. Therefore, measuring an EDM is a powerful 'smoking gun' for new physics, because any significant deviation from the Standard Model prediction would indicate undiscovered particles or forces.
The concept was introduced by Purcell and Ramsey in 1950, who proposed measuring the neutron's EDM. The first limits were set in the 1960s, and ever since, experiments have pushed the upper bound lower, with no detection yet—but each null result constrains theories of new physics.
To measure an EDM, physicists place particles (like protons, neutrons, or deuterons) in a strong electric field and look for a tiny precession of their spin, analogous to how a spinning top wobbles in a gravitational field. The challenge is that the effect is minuscule, so experiments use ultra-cold atoms, trapped ions, or molecular beams to maximize sensitivity. The corpus titles mention specific systems: the EDM of the tantalum-181 nucleus, the deuteron (a proton-neutron pair), and 'crooked electron' experiments. These measurements require extreme precision and control over magnetic fields, which is why they are often performed in dedicated laboratories.
EDM searches are used to test CP violation (the asymmetry between matter and antimatter) and to constrain models of particle physics beyond the Standard Model, which is essential for explaining why the universe has more matter than antimatter.
The current best limit on the electron's EDM, from the ACME experiment, is so precise that if the electron were scaled up to the size of the Earth, the charge separation would be less than the size of a single atom.
| a | variable | anomalous magnetic moment — anomalous magnetic moment |
| g | variable | Landé g-factor — lande g factor |
| \tilde{\mu} | variable | Corrected distance modulus or magnitude — distance modulus |
| \mu | variable | Original distance modulus or magnitude — distance modulus |
| A_D | variable | Amplitude of dipole anisotropy — dimensionless |
| \hat{n} | variable | Unit vector in the direction of observation — direction |
| \hat{p} | variable | Unit vector pointing toward the dipole axis — direction |
| \cdot | operator | Scalar product of two vectors — dot product |
| B(E1) | variable | Electric dipole reduced transition probability — reduced transition probability coulomb squared metre squared |
| Z | variable | Atomic number (number of protons) — atomic number |
| e | variable | Elementary charge — elementary charge coulomb |
| ⟨r²⟩ | variable | Mean square radius of the charge distribution — mean square radius metre squared |
| π | constant | Ratio of a circle's circumference to its diameter — pi = 3.14159 |
| δ(t) | variable | separation between trajectories at time t — separation |
| δ₀ | variable | initial separation between trajectories — separation |
| λ | variable | Lyapunov exponent — lyapunov exponent per second |
| t | variable | time — time second |
| I | variable | Moment of inertia about the axis — moment of inertia kilogram metre squared |
| r | variable | Perpendicular distance from the axis to mass element — distance metre |
| dm | variable | Infinitesimal mass element — mass kilogram |
| ∫ | operator | Integration over the entire body — integral operator |
