Physics:Nuclear electric dipole moment

The nuclear electric dipole moment (nuclear EDM) $d_{\text{e}}^N$ is an intrinsic property of a nucleus $N$ that can be non-zero only if CP violating interactions occur within the atom or the nucleus itself. They have been surmised to exist since at least 1963 in both light and heavy nuclei.

CP-violating properties and interactions that can contribute to a nuclear electric dipole moment include the neutron EDM, the proton EDM as well as CP-violating interactions of nucleons with meson or photons. At a more fundamental level, nuclear EDMs can originate from the electroweak sector, a non-zero $\overline{\theta }\text{QCD}$ or from physical processes that go beyond the standard model of particle physics.

Experimental Searches

Experimental searches of subatomic EDMs have nearly always been conducted with a powerful external electric field $\mathbf{𝐄}$ collinear with an external magnetic field $\mathbf{𝐁}$ that exploits the fact that the potential energy depends on the relative orientation of the electromagnetic fields

$U=-({\mathbf{𝐦}}_N\cdot \mathbf{𝐁}+{\mathbf{𝐝}}_{\text{e}}^N\cdot \mathbf{𝐄})$

where ${\mathbf{𝐦}}_N$ is the nuclear magnetic moment. The Larmor frequency is proportional to the potential energy which depends on whether the two external fields are parallel or antiparallel to each other. Flipping the direction of $\mathbf{𝐄}$ and subtracting the two corresponding Larmor frequencies leads to a result that is proportional to $d_{\text{e}}^N$. As this technique relies on large electric fields, it is always applied to neutral systems like the neutron or atoms making it difficult to apply to an electrically charged system like a proton or an ionized atom. Furthermore, applying this technique to neutral atoms suppresses the nuclear EDM contribution because the external electric field is nearly zero at the location of the nucleus as must therefore be the contribution of $d_{\text{e}}^N$ to $U$; this suppression is referred to as Schiff screening . In fact, for point like non-relativistic nuclei, Schiff screening makes the term proportional to $d_{\text{e}}^N$ in the potential energy $U$ disappear. However, atoms with large atomic numbers have significant relativistic corrections and relatively large nuclear charge radii. Hence, experimental searches of atomic or nuclear EDMs have traditionally used nuclear isotopes with large atomic numbers.

The best upper limit on an atomic EDM was measured on ${199}^{}\text{Hg}$, $|d_{\text{e}}^{\text{Hg}}|<7.4\times 10^{-30}e\cdot$cm (95% C.L.) using electric voltages of $\pm 6\text{kV}$ and $\pm 10\text{kV}$.

See also

• Neutron electric dipole moment
• Electron electric dipole moment
• Electron magnetic moment
• Anomalous electric dipole moment
• Anomalous magnetic dipole moment
• Electric dipole spin resonance
• Parity (physics) § Parity violation
• CP violation
• Charge conjugation
• T-symmetry