Abstract
This review presents the current status of experimental evidence for the occurrence of reflection-asymmetric or ‘pear’ shapes in atomic nuclei, which arises from the presence of strong octupole correlations in the nucleon–nucleon interactions. The behaviour of energy levels and electric octupole transition moments is reviewed, with particular emphasis on recent measurements. The relevance of nuclear pear shapes to measurements of fundamental interactions is also discussed.
Highlights
The atomic nucleus is a many-body quantum system, and its shape is determined by the number of nucleons present in the nucleus and the interactions between them
This review presents the current status of experimental evidence for the occurrence of reflectionasymmetric or ‘pear’ shapes in atomic nuclei, which arises from the presence of strong octupole correlations in the nucleon–nucleon interactions
I changes sign under T but d does not, so d must vanish if there is (T ) symmetry [44]. (d must vanish if there is symmetry under a parity transformation, P, where r → −r.) Under the assumption of the CPT theorem, if T is violated CP must be violated, and the observation of a substantial non-zero electric-dipole moment (EDM) would indicate CP violation owing to physics beyond the standard model (SM)
Summary
The atomic nucleus is a many-body quantum system, and its shape is determined by the number of nucleons present in the nucleus and the interactions between them. As with other many-body systems spontaneous symmetry breaking will cause the shape to be distorted from spherical symmetry and the nucleus to be deformed. The simplest shape distortion is quadrupole deformation (λ = 2) with axial and reflection symmetry, in which case the nucleus is shaped like a rugby ball (prolate deformation, β2 > 0). In the simplest case axial symmetry is retained, but, because β3 is non-zero, the nucleus loses reflection symmetry about the x–y-plane that passes through the origin (figure 1). It will assume a ‘pear shape’ in the intrinsic frame, either in a dynamic way (octupole vibrations) or by having a static shape (permanent octupole deformation). For earlier reviews on this topic, see [8,9,10,11,12]
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