Most nuclei are round or the shape of a rugby ball. But nuclear-physics theories predict that certain nuclei may have a more exotic pear shape, with more mass at one end than the other. Back in 2013, a team of researchers using the ISOLDE nuclear-physics facility at CERN showed that the nucleus of the radium isotope ²²⁴Ra is pear-shaped, making it the second known example of this special class of nuclei. The first was ²²⁶Ra, discovered about 25 years ago by a collaboration led by GSI. Other studies, at Argonne National Laboratory, have since claimed that a couple of barium isotopes could also be pear-shaped. A new study at ISOLDE, by the same team of researchers that investigated ²²⁴Ra, has now added another nucleus to this exclusive class.

To look for a nuclear pear shape, researchers typically measure the probabilities that certain transitions between nuclear states, called octupole transitions, will occur. These probabilities are enhanced in the case of a pear-shaped nucleus. This is exactly what the team at ISOLDE and other groups investigating nuclear shapes saw in their previous experiments.

In their latest work, and thanks to a recent upgrade of the ISOLDE accelerator system, which can now accelerate beams of radioactive isotopes to unprecedented high energies, the team at ISOLDE was able to measure the probabilities for several octupole transitions in ²²²Ra and ²²⁸Ra. From these measurements, they deduced that ²²²Ra has a stable pear shape, while ²²⁸Ra instead oscillates between a pear shape and its mirror image.

“Our results allowed us to conclude that so far there are only three cases in nature – ²²²Ra,²²⁴Ra and ²²⁶Ra – where there is incontrovertible evidence for nuclear pear shapes,” says the principal investigator of this study, Peter Butler from the University of Liverpool in the UK.

So, why is finding another nuclear pear shape interesting? The more the better, because these exotic nuclei are useful for testing existing nuclear theories. What’s more, they could be used to search for an electric dipole moment (EDM) in particles.

The EDM describes the separation of the centre of charge from the centre of mass of a particle. The Standard Model of particle physics predicts that it should be non-zero but very small, but theories beyond the Standard Model generally predict a much larger value. In addition, if a nuclear EDM exists, it should be easier to measure it in pear-shaped nuclei. Therefore, nuclear pear shapes could offer a sensitive means to test variants of the Standard Model and probe new physics phenomena.